Water leak internal - Jacuzzi 7.5 GPM 180000 BTU Internal Gas Tankless Water Heater Natural

excessive internal pressure or just a bad plug. Try replacing the plug.

leaking is a sign of rust internally
cheapest fix is a new hot water system

Check all external pipes, includeing the ones at top. If you find no pipeing leaks, the odds are, depending on age and waranty, you
have an internal tank leak and need to replace. There is virtually no way to fix that kind of leak.

Replace. There is no fixing an internal leak

It's not screwed on good enough or there is a seal out of place.

I wrote a page on leaking water heaters.
With a checklist.
It sounds like you've done quite a bit already.
The element can leak around the gasket.
The TP valve can leak where it screws into side of tank.
Otherwise a leak on side of tank sounds like the tank is rusted through and needs to be replaced.
http://waterheatertimer.org/Water-heater-leaking.html

You should turn off electricity and water to the tank, and open tub valve on hot side to relieve pressure so the tank will not rupture.
Then you can limp along with a failed water heater by turning the water and electricity on only when hot water is needed.
This will buy you time until a new heater is installed.
A typical electric water heater produces 21 gallons of hot water per hour.
A shower uses 6-9 gallon of hot water, so heater only needs to be turned on 20-30 minutes for a shower, more if the incoming water is very cold.

To economize with your next heater, you might read the following two pages:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html
http://waterheatertimer.org/Replace-anode-rod.html

make sure that the water is not coming from the over flow tube or the drain valve at the bottem of the tank first. If it is not then yes there is an internal leak and the tank should be replaced

It's time to buy a new water heater.

replace