A diagram of the lower unit for a 1956 Ward/Seaking outboard motor

Question edited for clarity, but needs engine model and HP.
Question moved from Dishwashers.

Some ideas here. Check that the impeller spins (the drive is okay), but it sounds like you have blocked passageways.

https://www.google.com/search?q=1993+Mercury+Outboard+Engine.+Why+is+it+not+peeing

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Hey Mark,
Find your engine from this list.. then continue until you find the correct parts diagram that corresponds to what you are looking for...More info would help.. serial number, ID number. any numbers on any plate..
Find Mercury Marine Outboard Motor Parts by Engine Model

No, the gears in the lower unit are cut to rotate in the "other" direction... If the impeller were to rotate opposite - it would mean the engine had to run backwards... and that does not happen.

Have you replaced the impeller in the lower unit recently? If so was it in pieces? I would start there.

Look Here!
http://www.boats.net/parts/search/Suzuki/Outboard/1989/DT115/parts.html

Find Mercury Marine Outboard Motor Parts by Engine Model

put it in forward gear. Drop the lower off by releasing the bolts and nuts holding it holding the lower on. (one is hiding under the back pick up screen)- some models had a plug in the back top of the exhaust/intermediate housing hiding an allen head bolt pointing down.

Once released, put a protector in a vise and clamp the lower to the bench.

2 screws or bolts will hold the water pump cover on. (the drive shaft runs thru it) Clean the carbon and rust off the drive shaft will allow the pump housing and impeller off the lower. Retrieve the funny looking key drive which runs the pump.

Inspect the shaft, the lower unit oil for water. and find all the pieces of the water pump impeller which may have gone places they shouldn't have.- including the outlet tube from the power head. Check this by putting a tube on the end and blowing- you should hear and feel the pressure in the exhaust and tell tale hole.

Now take the model number , the old pump parts to the marina for a replacement. Do a side by side comparison at the marina so you get the right stuff. Check the inside drive of the rubber impeller and see if they are exactly the same.

NOW- put the new wear plate / gaskets and drive key on, hold in place with grease. Slide the impeller on the key and spin the drive in both directions to be sure it has the impeller and will securely turn it.

Put grease in the cup of the pump housing to prevent dry start and help seat the impeller blades. Spin the shaft as you lower the housing in the pump so the blades all face in the right direction (clockwise). Screw in the bolts. Grease up the seal the watertube goes in and the spline end of the drive shaft.

Up the hole it all goes, now you only have to spline the shifter (it hasn't moved has it?) The drive shaft and the water tube all at the same time.

Check the shifter for correct position and bolt it down

sounds like you need a new water pump impeller.
to access the impeller the bottom of the motor needs to be unbolted and the impeller is fitted to the shaft, it is keyed so it only goes on one way. ( impeller kits will come with new gaskets,grease.O-rings and a new key ). re-bolt the lower unit and test.

c...

Okay this is on total memory. The shift shaft for a lot of those engines goes right up through the pilot shaft and exits just in front of the block under the carburetor(s). You will see the **** linkage arm connected to this shaft sometimes it's double nutted. See if you can slide the lower unit back up to the housing and put a bolt in it to hold then undue the shift shaft linkage, Remove the bolt and drop the lower unit. Be careful of the water tubes and check make sure the rubber grommets are in place in the pump housing not on the tubes. Clean up the pump housing and install the new impeller kit.
Put the impeller down the shaft on top of the impeller plate, lightly grease the inside of the housing the impeller plate and impeller. Then lower the pump housing down to the impeller while turning the drive shaft clockwise lightly push down on the impeller housing. This will fold the impeller blades all in the correct position and provide lubrication to the pump until the water gets into the pump. To make life easier re-grease the drive shaft spline and the water tube grommets this will ad ease in reinstalling the lower unit. be careful to make sure you thread the drive shaft, water tubes and shift shaft all in at the same time. After installation you may find you need to adjust the **** linkage.

i have fought with mine in the past also.
put lower unit and the control in neutral
remove all spark plugs and have a helper turn the fly wheel clockwise while you install the lower unit i had to use channel locks to put the lower unit to neutral
good luck i hope this helps