Like any other system, you need to look at the draw/usage to determine the input/charging requirements.
Will the bilge pump be running constantly during heavy weather, and be solely responsible to keep the boat floating? Or does it just empty out a tiny bit of water during the summer?
If the former (or similar), you'll need a full-on charging system, but may have to also run from shore power during the winter or storms. If the latter, then the cheap trickle-chargers may work just fine.
"The enemies of reason have a certain blind look."
Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.