AFAIK, the manufacturer only states the fuse to be rated at 25A and is silent about the switch rating??
The fuse protects the wiring (so wiring must stand 25A, which is a silly thin AWG16) and the switch must withstand current at the fuse rating. Read on, here it comes: A 20A switch is rated at that value because you can use it to break
a circuit carrying 20A many times before the contacts burn up. But when the switch is just on, it can handle much more. As long as the pump draws 20A or less when running (no locked rotor amps or start-up amps are relevant) you can use a 20A switch for it. Short overloads like during start-up are provided for in switch ratings. Get the full specs if you don't believe me (it's called intermittent and continuous ratings for 3 states: switching on, being on and switching off. The latter one is the lowest rating because switching off causes more arcing)
I would never ever use a relay or solenoid on a bilge pump
. The failure rate of these are just too high for this crucial device. If you really want to over-engineer it, use a bigger switch, no matter what needs to be done for that.