Sounds like your vent tube may not lead up high enough before going down & out. They''re usually led up to the top of a hanging locker, near the cabin top, before turning down to go overboard. If the vent us ending up underwater, it could also be siphoning
fuel out the same way an
exhaust loop that doesn''t go up high enough can siphon seawater into your engine (and sink you) when you''re in reverse or a high following sea.