If you remove the floorboards and the underfloor in the master bedroom at the back, you'll find something like this. There are six of these shallow compartments, built with small fiberglass walls to strengthen the floor and the hull that is very flat/weak in this section of the Baystar.
The draining of the stairwell at the back is led through one of the hoses and into the bilge of the keel which is situated further forward. This works as long as the inlet in the stairwell is not blocked by leaves or other rubbish.
The problem is the water coming from condensation etc that actually gather under the floor here.
The compartments are inadequately drained, and a significant amount of water seem to build up without anywhere to drain out, even if the water can travel between the different compartments through small holes in the small walls. The problem was known to the Emerald Star in the hiring period, but they did not reach any final solution.
Suggestion: Build a nice, nearly invisible hatch in the floor beside the rear bed and put an automatic bilge pump in at the lowest point, it will remove 90% of the water.