An automatic one will have the possible advantage that it won't run out of water, but a non auto one might be smaller and possibly cost less as well. The bearings might not be happy being run dry if too much bench testing happens. By their nature, bilge pumps are unidirectional, you don't want them sucking water in. In the old "Supermodels" series a few years back, I recall that they used a reversible pump to flood/drain HMS Fearless. The details escape me, but there might have been a drill attachment involved.
Looking at the numbers, the bilge pump motor pulls about 48 Watts (4 amps on 12 volts), well within what you might expect from a fairly ordinary 540, but a lot depends on the efficiency of the pump. Mains drills, on the other hand, tend to start at 350 Watts (memories of the old Wolf Sapphire) and work upwards, looking at the labels on B&D and Skil drills.
Looking closer at the innards of yours, I suspect it isn't truly centrifugal - it doesn't work by throwing water outwards, but the vanes in the hub get flung out and swat the water in one direction or the other. Sort of a well enclosed paddle wheel.