Whale 220 and 320 are both diaphram pumps, meaning you can install them out of the bilge
, and well above any bilge water
level, hmmm, the sensor need to be down below. Just run the suction hose from the pump to the lowest part in the bilge. That hose does not have to be big in diameter, as the capacity of these pumps are relatively small. These pumps suck the bilge to near-dry, something a centrifugal pump will never do.
There are other brands of diaphram pumps ie Johnson, Viking.
A second pump can then be mounted higher; maybe select a high volume centrifugal one (with a large diameter hose). And if nothing goes wrong, this 2nd pump and water
sensor/float-switch will never see any water. So it pays to test the sensor/float switch once a month!
Claiming bias here: this kind of setup have served me well over 20 years on 2 boats, never had a bilgepump or sensor fail..... sofar. Ooops, had a sensor on a Whale 220 gulper fail once, I thought...... it was a faulty connection. Not on the bilge unit but the one on the shower sump