1990 Sport Nautique Problems

I would strongly recommned doing a compression test to see how your engine health is.. from there start looking for the causes of lack of power ect...weak power can came from lots of reasons..from problems with fuel(system or fuel quality), ignition(timing),etc, to the worst that would be engine wear, piston rings etc......wich engine do you have? base or proboss? Is the boat with ballast when pulling rider or just people? how many hours?

yes i thought of that after reading again your problems....maybe you have a clogged fuel line or at the tank some debris floating around that clogges the fuel. But in that case the sympton would be like the engine will stall, strve for fuel... shut off...when you feel it w/o power you feel its running rough or its just the loss of power.. it eventually shut off?
hows your oil?
I would stick with the compression test anyway... and then from there star trouble shooting....

Eric 11.5 volts is not enough with boat running should be in high 13s depending on battery condition.
Sounds like a bad connection somewhere. Does the engine turn over on the starter strongly?

First stop would be to clean connectors at engine harness plugs at rear of engine. Then work you way back.

The dash wiring is not the best in these boats so the voltmeter always reads lower than it really is. If you have a test meter check voltage at battery when the engine is running and also when it is stopped this will give you a indication if the charging circuit is working and a truer indication of battery voltage.

The low voltage is a problem but do not rule the fuel system out. Without starting the boat give the throttle a couple of pumps and see if your accelerator pump is squirting fuel down the primary throat of the carb (remove flame arrestor to check). If you see a good solid stream there is fuel in the fuel bowl which is a good start diagnosing fuel supply problems.

Hope this helps get back to us with the results and resolution.