As GI Auz said, that box is the shower sump that collects water from the sink/shower, etc. The water should be contained in that box and be pumped out. My boat has one as well but the pump inside the box wasn't working so the water would overflow out of the box and end up in the forward bilge area. My forward bilge pump (separate from the shower sump) wasn't working either so it didn't pump out the water.
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Terry
Current: 07 Crownline 255 CCR cuddy - 350 Mag MPI/B3 "Casi Cielo"
Previous:97 245 Sundowner 5.7GI/SX
Tow vehicle: 1986 GMC 2500 Camper Special 454/TH400
