its in both hulls....difficult to have a leak on both sides or one that distributes evenly...each hull
is independent , has its own water heater
, etc i suppose... so its unlikely to come from the water system.
I would check the stanchions near the head
which you say is wet on boat
sides. If these are constantly used to tie fenders they can get loose at times or loose their seal.....and then rainwater comes in.