Solent, that is done on some full scale float planes, normally with a pair of retracts on each float, one at the nose and one at the step of the float. I've seen videos of what happens when the pilot forgets to retract the wheels before landing on water - it sure isn't pretty, but certainly very exciting! It most often results in the plane stopping so suddenly that it turns turtle, often with fatal results for the occupants. It would be very cool to do the retracts on a model, but remember, wheels up before water landings!
Yeah, wheels or open wheel wells in the water cause huge huge drag issues.
Your wheels will need to retract well out of the water. If they retract into the float, then they must have doors that seal fairly well or you wont even be able to take off and - as mentioned above - landings get a little too exciting
Yes, exmech2k, that's the same setup I've seen, but commonly on Beavers, in my area. Several years ago, a Cessna like the one your link shows touched down at Cultus Lake, a popular summer resort/camping area about 15 miles from my home, with the wheels down, with disasterous results, both pilot and passenger dying in the crash.
Yes, nice and simple like that but look at the size of the wheels compared to the airframe.
At model sizes you would need a very smooth runway indeed.
It would be difficult to get 'rough grass' size wheels to retract out of the way with this sort of installation.