If you have room on the front of your engine to clutch
a pump, they are superb. However most of us do not have that room.
The ones mounted on the prop shaft have three serious drawbacks. First, most of us have about 3:1 reductions so the RPMs are much lower and thus the capacity is greatly reduced. Second, you have to be motoring pretty fast in forward gear
for them to work, which most of us will not want to be doing if you have a significant hole in the boat. And third, most of us don't have much room back there either.
If you have a decent battery
bank and a large inverter
, AC pumps can move a huge amount of water, and can be stowed anywhere and are "relatively" simple.
Portable gas or diesel
run pumps (trash pumps) are required on most coded vessels, and can most a lot of water. But they need to be regularly used and maintained, which they often are not. And they are pretty big.
A combination of the above two . . . AC pump plus say a Honda
2000 gen is a workable hybrid solution. The Honda will get used in its normal gen/battery charging
role, so will be used and maintained and actually run if needed in an emergency.
The normal dc electricity bilge pumps are ok for a trickle, but have no where near the flow rate to deal with a real hole in the boat. It's worthwhile for any skipper/owner to look at the inflow rate of a 1 1/2" hole in the boat 2' underwater and compare it to the typical electric bilge pump flow rate. It's eye opening. If you are depending on typical electric bilge pumps your priority MUST be to immediately find and plug
(as well as possible) the hole. You need to get the inflow down to the "fast trickle" category ASAP. This is different from the above true "dewatering pumps" which can actually keep up with a relatively serious hole. . . . But of course you still want to plug
it ASAP but they give you some time to do that and to deal with any other serious problems that may have occurred (like an injury).
water">engine cooling water pumps do not move much water . . . Even less than the bigger dc bilge pumps. Personally I would prefer not to potentially compromise the engine (by possibly sucking trash into the cooling water system from the bilge) for the relatively small gain of its pumping capacity.