burnt contacts due to voltage spike.

You have a permanent magnet motor spinning when the switch opens. The motor instantly becomes a generator with no load on it. The voltage spikes as the contacts are separating. A spark burns the contacts. No pump switch maker addresses this. This is the same issue that causes relay contacts to fail.

My solution is to put an automotive 30 amp headlight relay (cheap) in a socket between the switch and the pump. If the relay fails replace it - about $5. Have not had a bilge pump switch fail in years.

A relay also generates a spike as it opens but it is much easier to deal with than a rotating motor. A diode in the reverse direction across the relay coil with a .01 mfd capacitor across it absorbs the reverse spike from the relay opening.