Heater element only gets 120volts. Should be 240v, correct? Water does not heat up.

you can its your water heater , but the heating elements are sized to tank.. Stay with original specs The electrical may be to small for the additional power requirments

most water heater of that size is rated 240v, therefore it needs 2-pole. is it 240v rated? make sure you have a properly sized circuit breaker & matching conductors (wires) with the kw rating of the water heater

The heating elements in most electric heaters for water are sequential. That means that both are not on at the same time in some cases. There are individual thermostats for each set of heating elements. So you should check the integrity of both the heating elements with a meter and the thermostats.

Make sure wires are removed from element when ohm-testing.
http://waterheatertimer.org/How-to-test-water-heater-element.html
Watt and voltage rating of element appears on end of each element.
Volts squared divided by watts = correct ohm testing.
Two-element water heaters are rated for 240Volts, but can be wired for 120Volts.
Check that water heater is receiving power from circuit breaker box.
So if breaker is 2-pole 240Volt, then check that heater is receiving 240 V across top two screws on upper element.
Replace upper element.
http://waterheatertimer.org/How-to-replace-water-heater-element.html
Other resources.
http://waterheatertimer.org/Test-electricity-to-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Marathon elements are specific to that water heater and can be ordered online, but generally are not at the local hardware.
Good to keep a couple spares around.
Marathon is 240Volt water heater.
Thermostats turn off one leg of the 240Volt circuit, so elements and themostats will always test for 120volts.
You removed wires off elements and tested each for continuity and tests showed elements to be good.
Test top two screws of upper thermostat and make sure you are getting 240V to the water heater.

If water heater has some hot water, then that says water heater is getting 240V, upper element is good, and remaining suspects are bad thermostats, bad lower element, or sediment build-up, or combination.

Open following link for troubleshoot run-down:
http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html

Water well.
If you have sediment build-up that reaches lower element, then water does not circulate around the element and water heater no longer heats very fast or accurately. If sediment is the problem, sooner or later the lower element will burn out.
Doesn't hurt to flush out the tank now and then, and vacuum out the sediment if that is problem.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

Sounds like water heater is getting 120V but not 240V

You say there is no hot water inside the tank?
You have two elements, so tank is 240V.
Both elements test for continuity.

These three symptoms together imply that water is getting 120Volts instead of 240Volts.
Water heater appears to have electricity, but is receiving power from only one leg of the 240Volt circuit.

Open following link for troubleshoot of electricity to water heater.
http://waterheatertimer.org/Test-electricity-to-water-heater.html

Upvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Water heater has to be full of water before power is turned on or elements will burn out.
If upper element is burned out, then no hot water.
Lower element burned out, not much hot water.
If tank has 2 elements, it is 240V. Water heater circuit may not be delivering 240V. Test for 240V at connections.

Open following link for troubleshooting electric water heater.
http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html
http://waterheatertimer.org/How-to-test-water-heater-element.html
http://waterheatertimer.org/Test-electricity-to-water-heater.html

Add a comment with your experience so others with same-similar problem can benefit.

Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

If water is totally cold, then upper element is not getting 240V. Since element tested for 120V, then that says upper element is burned out, upper thermostat is bad, or circuit breaker is bad.
Best thing is to buy multimeter and do 30 minute troubleshoot:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Also read information below for understanding how water heater works.

Testing elements for voltage can be misleading.
If water heater has 2 elements, then tank is 240Volts.
240V water heaters have 120V on each element at all times.
So elements have hot 120V power, but they are not turned on until the circuit is complete with 240V.
Open following links to read specifics about 120V and 240V
http://waterheatertimer.org/B220C.html
http://waterheatertimer.org/How-water-heater-thermostat-works.html

240V is made from two 120V Hot wires from breaker box.
Each 120V line from breaker is called a 'leg.'
So 240V circuit has two legs, and both legs need to be turned on to complete 240V circuit.

Water heater thermostats turn off only one leg.
The other leg is Hot all the time.
So when you test screws on an element, for example test between a screw and any bare metal part of tank, then the screw will always show 120V.
That is just one leg of the 240V circuit.
But the 240V circuit is not turned On until both legs are connected.
Every residential circuit needs two wires to complete the circuit, and in the case of 240V, those two wires are both 120V.

When the thermostat turns on, it turns on the second leg to the element and that completes the 240V circuit.
http://waterheatertimer.org/How-it-works.html

Electric water heater can be repaired by homeowner
If I understand correctly, water heater is producing no hot water.
Reset button clicks in, but still no heat.

Three initial things can be causing this:
1) No power to water heater. Test across top two screws on upper thermostat for 240V. Red and black wire are attached to these screws. Screws are marked 1 and 3. Upper thermostat is same that has reset button. If power tests 120 across screws, then breaker may be off on one leg. If no power, then start testing at breaker.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

2) Upper thermostat is bad. Replace thermostat with generic upper thermostat available at hardware store.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

3) Upper element is bad. Test and replace element using Rheem 4500 watt element available at hardware store. Image shown below. Look at label on side of tank for element wattage. Rheem puts 4500 watt elements into all their 240V electric water heaters.
http://waterheatertimer.org/How-to-test-water-heater-element.html

If upper element is burned out the tank will not heat any water.
http://waterheatertimer.org/How-it-works.html

Rheem 4500Watt element


Upper thermostats are generic and interchangeable:

If original problem was caused by sediment burning out lower element, did you clean sediment out of water heater when changing lower element?
Two element water heaters are 240V.
Thermostats cut power on one hot wire, but 240V circuit has 2 hot wires. So lower element has power at all times coming from the red wire.
1) Use ordinary tester or multimeter set to 240V <> test each screw of lower element to bare metal part of water heater. Each screw should show 120V. If there is no power, then upper thermostat is defective, screw on upper thermostat is loose, or red wire between upper and lower parts of water heater is bad.
2) Remove wires from lower element. Use continuity tester or multimeter set to ohms <> test across both screws of element to see if power travels through element <> if element is bad, then continuity tester will not light up and multimeter will show no ohms. Put wires back on element after test.
3) Start with cold water in tank. Turn on water heater & upper element will turn on. Put ear on side of tank to hear if upper element is heating water. Test #3 screw to #2 screw on low left side of thermostat, should read 240V > Test #3 screw to #4 screw on low right side of thermostat, should read 0V.
Put ear on side of tank. When upper element turns off: Test #3 screw to #2 screw on low left side of thermostat, should read 0V > Test #3 screw to #4 screw on low right side of thermostat, should read 240V. If this test shows 0V and 0V, then upper thermostat is bad. If reading is correct, immediately move to next test.
http://waterheatertimer.org/images/Thermostats-numbered-divided-500.jpg
4) Turn temp on lower thermostat to 150. Using ordinary tester or multimeter set to 240V <> Test across both screws of lower element. If there is no reading, and you know element is good from step 2, then lower thermostat is bad, or wire is bad between lower element and lower thermostat, or wire is bad between lower thermostat and top part of heater.
5) Test across both lower thermostat screws, should read 240V.If this test is good, it means wires are good. Put jumper wire across both screws of lower thermostat, and test lower element again. If element finally turns on with jumper wire, then lower thermostat is bad.
Normal 0 false false false MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;}

http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html

If wire is bad between upper and lower parts of water heater, and water heater has tight-fitting 2" foam insulation, it usually indicates water heater replacement.