The main wire for the bilge pump and the wires from the switches in the in the heads for the shower drain are interconnected at a junction box behind the settee where the actual pump is located and run as one wire to the pump. If a float switch was integrated into the circuit, chances are that it was wired in a way that bypassed the shower circuit. Consequently, the only way that the pump gets energized is from the bilge pump circuit since the float switch breaks the connection to the pump until it is bypassed by manually energizing the bilge pump. In my 323 the wire numbers are 14 for the bilge pump and 162 for the shower drain. They run separately from the WAGO block where they are jumpered to the circuit breaker, and are connected together in the previously mentioned junction box. There was no float switch in the bilge when the boat was delivered from the factory. Wire number 162 needs to bypass the float switch.
Remember also, that in order for the shower to drain, the manual three way valve in each of the showers has to be turned to the shower drain position in order to work, then back to bilge pump position.