There was not a compelling reason to submerge the pump, and most things-electrical would prefer to operate high and dry, not underwater. The new bilge pump would be one of those big, ugly, noisy, diaphragm pumps that could mount in the cockpit locker, well above the oily water of the bilge sump. The electrical motor is not submerged. This just makes sense, as electricity and water don't mix well. You can't get the work done without some power being applied.
More power equals more work being done.
When these pumps are running, you can hear them. This would tend to decrease the occasions when the pump unintentionally was left running all day. Water isn't the only thing in a bilge sump. There's little bits and pieces of stuff floating around down there, the kind of thing that does not go through an impellor type pump very well, but can be sucked through a diaphragm pump with ease. Plus, the pump can run dry without damage.