GWS sells floats that you can add to planes like their E-Starter. You'll easily come in under $600. In fact, you can probably afford two planes if that's your budget.

Hobby Lobby also has a set of floats for their Potensky Eagle and Cobra. These can probably be adapted to other planes of similar size and weight with little trouble.

You could electrify a say, .40-size, seaplane for less than $600 if that price doesn't include radio equipment. An AXi brushless and speed controller combo doesn't cost much more than a high-end 4-stroke, and you can use CP2400 NiCds, which are cheap.