have you checked your water tanks
or rain or sink drains. shower
drains. leaky deck
enjoy your searching. at least fresh water doesnot sink boat
. may need to reweld any steel
tankages. could be your drains for sinks as you drain out water some sneaks out. could be a nice lil sneaky O ring at the deck
if it occurs when not using drain, then tank and hoses. connections or welds.