It's possible to have an exhaust riser pipe that is blocked by carbon build-up. The carbon can block the water, the exhaust, or both.
One way to isolate the water flow problem is the remove a seawater hose, direct it into a bucket, and then briefly run the motor. If you have good flow, move down stream and try again. If not move upstream. Do this until you isolate the problem. If you don't want to get seawater on anything hookup a hose or bucket of freshwater for the tests.