Typically the bilge pump should be wired as "always hot". Meaning if the battery
switches are off the pump still has power applied.
Most modern panels
have a seperate circuit set up for this and "memory" circuit for electronics
that require it.
My pump is on a totally seperate fused circuit not connected to the dc panel or switches in any way. Both positive and negative go straight to the battery
bank with the bilge control panel in series on the positive side.
However, no problem with the negative going to the engine ground.
But, does your pump work with the battery switch set to off?