Are there lockers at the headboard and footboard? Is it wet inside of there as well? Rubrails, rail stanchions, cleats, cokpit lockers can all leak over time. I'd look around the water tank and look up. It might be leaking in the area of the water tank, then once on that section of sole, run and wick it's way into the next compartment.
Water travels mysteriously on a boat. What is wet is almost never where it started. It'll move in all directions but up.