Need to control 100 Watt Pump With 15 Watt Float Switch
Use the float switch to energize a relay.  An automotive headlamp relay would work fine, normally have 12 volt coils, and can handle upwards of 25Amps current easily.

So you have 12V -> Switch -> Relay Coil -> 12V Common for the switching circuit,
and then you have 12V -> Relay Contact -> Pumps -> Common for the controlled circuit.