What I do, and I've done this since VV1, is watch a villager that I drop in the ocean. If they look like they're going to get out of the water (after awhile, you can tell very quickly by the direction that they swim), I'll pick them up and drop them in the ocean again. That way they will stay there and fish (like I want them to). As Warren has pointed out, this doesn't apply when the villagers initiate fishing on their own.