most switches want some current through them to break up the oxide layer, etc. Running them below about 15 mA will actually shorten their life! (glass-encapsulated reed switches are one exception; mercury switches (and their cousins, mercury-wetted switches) are another. ) CAVEAT: I wouldn't want mercury anywhere near water that I might use for drinking or bathing. . .
there are switches that are rated for DC use; microswitch(r) is one such brand. They market some that will switch 125 VDC and various currents, which will serve our purposes quite well as long as they are installed in a 'weathertite' enclosure. (they're specified for 1 million make/break cycles at rated capacity. . . that's a LOT of pumped rainwater!) Figure about $60 for one equipped with the enclosure and a lever that can be fixed to the float rod. Pricey, but you'll only ever buy them once, especially if you put a moderate-sized ceramic capacitor across the contacts. (100v, 1uF per amp or so)
Or you can get a cheap switch that will handle 1/4 amp DC or so and put a cheap ($15) relay rated for 10A 125Vdc per contact next to the pump. (the potter and brumfield 'krpa' series is one). . . makes things easy from a repair point of view, but uses about 1/2 watt more power than the straight switch while the relay is pulled in. You might make that up in shorter high-current cable runs, though.
if you're a little handier with the soldering iron and electronic theory, a virtually foolproof 'forever' switch can be made with an LED and a Photodiode or Phototransistor, which can then drive your IRF 510 or 610 MOSFET. In fact, some surplus/ supply houses (jameco) sell 'photointerrupter' sets out of printers, etc., that are already set up for this sort of thing. Just be sure and test it before potting the electronics in polyester or epoxy resin to keep out water. . . (voice of experience) The price is certainly right; about $10 including the fet and [cheap] epoxy. figure about 5 mA of 24/7 current draw if you go this way. . . but the right FET can drive the pump with no intermediate relay. (International Rectifier sells MOSFETS that will handle 10+ amps in a to220 package, and you can gang the FETS togeather for any desired current flow. again, the FETs can be right next to the pump, reducing the need for long runs of high-current cable.
ghurd's point about getting the pump that handles the most gallons per WattHour is well taken. . . .
-Dan