I have the yamaha cover and use it whenever I am away from the boat for more then a day or two, it has not held moisture and it drys very fast if wet. It is treated to repel water, and the inside of mine has not gotten wet or held water that I have seen since it has a draw string to pull it tight around the engine, plus it tightens where the engine is tapering down so water is already dripping off before it gets here. As long as the cover does not rip you would be fine. The cover might need to be reteated over time with a water repeleant product that would be used by sunbrella or a similar fabric to keep it water resistent. Mine have been on the boat two seasons now and still repel water, plus they keep the decals from fading and the engine shinning much better. I have not had condensation on the engines on cooler days where it gets wram or vise versa either. I have no complaints about them honestly. You should be fine using one, a bunch of guys in my marina use them and if you look, their engines and mine all look much better then any others in the marina. I'd use them if I was not using the boat for a few days at a time, especially if a newer boat/engine.
