I realize this is an old forum.. but I have had the exact same problem. I left the sea valve open for a couple weeks and then started up the engine after a couple tries. We motored about 200yds and then the engine all of a sudden sounded like it had no power.. and the throttle did very little. We just made it back to the dock before it died.
Upon inspection, there was water in the carb and so we followed all the steps here about putting MMO in the cylinders and cranking her a few times... draining the carb.. etc...
But after the carb was dry and everything was put back together... we cranked the engine... with no luck.. and then saw that water had filled up the carb once again.
Anyone have any ideas as to why the carb keeps filling up with water after i try to start the engine? (Atomic Four - raw water cooled)