Fresh, running water from streams and stuff is usually alright. Lots of natural filters, and the motion keeps things from stagnating. With oceans, you you have to imagine that they team with life. Tons of life, all hunting, eating, shitting, and mating. All the time.

Plus salt water dries out your skin terribly. You should at least boil it. In my time, most modern ships have systems to desalinate and sterilize seawater.