Yeah...don't get the cub for your first model.

Get a kadet or a senior kedet.
Get a .50 size engine for it...so this way when you go to floats, you'll still have power to spare.
A kadet is bigger than most trainers with plenty of wing area for lift.
A senior kadet ..while still in the .40 size range has a big wing....80"

I dunno.. maybe look into those Balsa USA model, later (North Star). It's jet looking but you can fly it off of water.

Maybe look around for some retrieving boats plans too...this way you can send it out to retrieve your airplane
if the engine dies or something. You can probably make it out of PVC pipes and some lite ply.