It happened on the girl friends suv, exactly what your boat did and she called me telling me the idle was going up and down and it finally evened out but give it gas would die but start right up with idle up and down then settle down, told her to let it warm up to temp and see what happened and still wouldn't take gas so she shut it off and sat there for 15 or 20 minutes till I could get there, started it up then and it worked fine. I at first thought water in gas or water froze in fuel line but when I hooked up scan tool it set 2 codes, 1 was IAC other was fuel actuactor, Hasn't done it again. A buddy and me went down to his boat a while after that just to check it and he started his engine up and did same thing, checked intake and they were iced up on outside, same deal let it warm up and everything was fine. Just a fluke with the right weather conditions. I would think the odds of your 2 engines with separate fuel tanks to develop a water in fuel condition at exactly the same time would be pretty high!