Don’t use the built-in mic. It just has no presence. The only time I have used it and liked the effect was when i shot a funk band reversal while surrounded by the musicians. It is truly lousy for recording voice unless you are right on top the person talking. And even then it’s marginal.
When you’re on the water, it’s actually rather noisy. The sound of the waves, the hull smacking them, motors running, and the wind. And yes, use as much protection from wind as you can afford. Even if you have to cut a hole in a sponge and stick it on with rubber bands. I recommend you use a good quality directional microphone like a Sennheiseer 416 with a big furry thing on it (like a Rycote Winshield). And to be safe, put a wireless lav with windscreen on your subject too.
Use a headset and you might find it useful to go below, out of the wind, and playback bits, just to monitor what you are getting, Of course you might get drop dead beautiful day with no wind and fantastic sound with no effort, but every time I’ve been shooting on the water in a “boat” (as apposed to a “ship”) it was, shall we say, “challenging”.