Goldberg makes a set of floats specifically for their Cub which are a great match. An OS 70 will be perfect.

The float kit also includes a lower Fin to add to the rear of the Cub which is helpful.

Set up the floats so that the wing sits with about 2 degrees postive incidence when the tops of the floats are level.

You'll also find that a good deal of lead will be needed to the forward tips of the floats to get the CG back where it was - Add this weight to the FLOATS and not the plane so when the floats are removed, you don't need to re-balance.

Take offs from water are not that much different, other than the fact that you really want to track straight and keep the wings level in order to avoid having one float dig in.

In the air, you'll feel a definite sluggishness due to the extra weight and drag, but nothing you won't easily get used to (Just make sure you have LOTS of altitude before attempting a loop or roll! )

Oh yea... Be sure to put your Rx in a plastic bag, and make sure there's a boat handy for retreival if necessary (And it's often necessary)

Good Luck