Check your engine oil level also... the oil level should be right in the middle of the bend on the dip-stick with the watercraft sitting level. If it's near the top of the bend (or above the bend even!) then there's too much oil in the crank you'll need to suck some out with a suction pump. Fun, fun!
Also with the seat off and the engine on a waterhose for exhaust cooling, you can run the engine and reach down and squeeze the SC to carb air hose... as you goose the throttle you will feel the hose expand under your grip as the SC makes boost. If you feel nothing, then the SC isn't making boost something's wrong with it. When connected to a waterhose remember the rule: Engine on THEN water on, water OFF then engine off! No water should be flowing while the engine isn't running. Don't flood your engine with water from a hose by accident (it happens far too often to people)!
You changed the plugs but how old is the gas in the tank? If it wasn't treated with stabalizer last Fall before putting it up for the winter then the gas may be bad. Bad gas can also cause the symptoms you describe. That's all I can think of myself right now.
Regards and good luck!
- Michael