Depending on your panel manufacture, typical panels
have a total combined load of about 100A, distributed across each main bus. The buss bars in mine (a Blue Sea Systems panel) can each handle about 100A per row without any problem, but the cable I have going to the panel is 4AWG, so I am limited by that cable rather than much else. A 20A breaker isn't that big of a deal, the real issue is what ELSE are you running at the same time.
pumps that large have a float switch with aux bypass and you hardwire the motor
(fused, of course) to the unswitched power supply from the batteries. The panel simply leverages the built-in relay that the float switch also uses.
I don't know if this is an auxiliary bilge pump
or your main, but if its your main pump, you should definitely have it on a float switch / water
level sensor / similar device and otherwise fused directly off the main power leads. That way when you shut off your main breaker panel you don't lose the pump. The main panel can then provide a circuit AROUND the water
level sensor to activate the pump manually should you want to before the sensor would, or if the sensor fails. Redundancy = good.
If its an aux pump, I'd vote the relay route
mostly because I like keeping the current draw off my panel as low as necessary and fusing the higher current devices off a separate buss bar/fuse panel I use for that purpose. 20A isn't a super high current device but its not a bad idea to have all your pumps similarly wired, plus this is a very good way to guarantee that any voltage drop across your panel won't compromise the performance of your pump.
So either way, my vote is make the panel carry the least amount of current necessary to do its job and fuse the pump's main supply off the unswitched power supply lead. The specific manner in which you do this can vary.
Just my 0.02.