[Bruce_Sailboatdata]

This isn’t something that can necessarily be answered here. Too many variables. I’m more of a small boat one-design sailor. That said, your head probably has a Y valve that can direct waste to either a holding tank or directly out through a through-hull into the open water. This valve should always be switched to directing waste into the holding tank (unless you are offshore) and the through-hull closed. Could that valve be open allowing seawater to back in?