As someone who flies with floats regularly, i've often wondered that myself.
Having never had the opportunity to do it, i'm not too sure, but i'd assume you'd want to get as close to zero/zero as possible, although I saw a video of an R44 doing them to water, and it seemed pretty forgiving even with 10 knots or so of forward speed - just a big splash. I'd rather do one to water than land. (don't have to worry so much about making the spot in the ocean!)