The way I have built mine is to simulate a retractable gear, I can take the wheels out for on water ops. However, I had thought about sinking-in the wheels enough so they could stay as a fixed gear. Should my plane actually fly, I plan to experiment with that or a skid plate as Himat suggests, further down the road.Very narrow (1" wide) fixed wheels should work for water and paved runways.
The narrow wheels slip through the water like a sailboat daggerboard.
I guy named Reynolds built RC models with fixed wheels on floats made from clear plexiglass.
I will be doing this when I get around to building floats.