Depends where you are. NEVER off a river mouth or estuary no matter how far off shore you are (I've seen shit from the Amazon, for example, 200 miles out to sea!), and the world's biggest industrial sewer spews it's pollution out of the Cabot Strait. 20-25 miles out on a Continental shelf, depending how murky the water is (eg east coast N America), 5 - 10 miles out where the shelf drops off (eg Hawaii or S Africa), never in enclosed waters, eg North Sea, Baltic, Med, Sea of Japan, Yellow Sea, Persian Gulf, Great Lakes etc. Talk to the boat driver about water depth, current proximity to land etc. He (or she) will probably know these things.
Discourage incest, ban country "music".