Carbs and fuel filters are finicky beasts. A clean carb can get gummed up in short order if gas is allowed to sit in it. Changing the fuel filter is an easy and cheap attempt at a fix. You might luck out and have a clogged filter restricting fuel flow.
Spray cleaners down the carb generally can only hit the orifices in the carb throat. Once in a while they have worked for me. Usually on 3 and 4 hp mower engines. I did have a motor recently where the gas had evaporated in the carb of a 2-stroke outboard to leave a heavy coating of oil in the carbs causing a rough running engine. Spray cleaner did help that motor.
'In-Tank' Fuel system cleaners have bit me in the past by loosening up everything in the fuel system (Tank, lines, fuel bowls) and clogging the carb jets (and taking out the in-tank fuel pump on a car). My local mechanic bitches about modern fuels older than 30 days old not being too stable and the problems that older fuel can create. A good habit is to put fuel stabilizer in keep the gunk from forming, and sticking and clogging the carb orifices, as well as preventing water from accumulating in the gas. And running the carbs dry after every use helps prevent gunk in the carbs.
You could have a sensor issue. The gauge apparently says you have more oil than you actually see in the tank. My ski and 3 2-stroke Japanese outboards will limit the rpms if the oil supply is anything less than 25% Full. What's odd is that the gauge and the limiter should be using the same source (oil float). Maybe the ski is in a limit mode and the display has an issue? A quick test would be to fill the oil tank to see if you suddenly get power back. You do need to determine why you are getting different readings from the dash indicator than what you are seeing.
Speadfreak99 recommends doing a compression check. That is to determine if you've lost compression on one or more cylinders. A bad sensor could contribute to the motor running lean on oil, causing ring, cylinder, and/or piston damage in one or more cylinders. You can get a compression gauge that screws into the spark plug hole from most auto part stores. You can Google how to use it pretty easily too. In short, once you get your compression numbers, they should be consistent across all cylinders. Compare that number to the manufacturer's numbers. They should be pretty close. If one cylinder is drastically and consistently different and is much lower, that indicates a mechanical failure. Next stop is the shop.
Water in the bilge can be anything. Loose or bad hull plug seals, leaking hoses, etc.
As always, I defer to the more experienced posters on this board. They know best. My experience is generally based in a variety of 2 stroke and 4 stroke applications in other types of machines.
Good luck,
Greg