Suppose I'm on a private yacht. I'm in international waters, and I forgot to bring a trash bag. So, I go ahead and just dump all my crap into the ocean. Is that illegal?
The Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter is probably the right starting point:
The purpose of the Protocol is similar to that of the Convention, but the Protocol is more restrictive: application of a "precautionary approach" is included as a general obligation; a "reverse list" approach is adopted, which implies that all dumping is prohibited unless explicitly permitted; incineration of wastes at sea is prohibited; export of wastes for the purpose of dumping or incineration at sea is prohibited.
However, this is probably one of those situations which is difficult to enforce any action against an individual working with small amounts of waste. Necessarily, some items will be lost overboard accidentally.