Shower drains there, too
The 28.5 has a segmented bilge. I assume you're talking about the lowest one, where the pump is. There are 2 areas that drain there: the ice box and the head. If you take a shower or if the head overflows, the water will drain to the low bilge. If the water is in the upper bilge compartment (accessed by the aft bilge hatch, it's likely spilling over from the engine pan. Water will accumulate in the engine pan from either a leaking stuffing box or a leak somewhere in the cooling water inlet. Lastly, keep in mind once there is water in the bilge, the pump cannot empty all of it, as the contents in the hose between the thru hull and pump will always drain back once the pump shuts off.