Ideally, you want to balance the plane with wheels (adding weight to the plane). Then you add floats and re-balance by adding weight to the floats. This way, any time you switch, the plane will be balanced.

In your case, I would still add the weight to the floats since it will take lees weight because you can add it so far forward. Usually a few stick-on weights added to the front of each float works (If it takes 4oz, add 2oz to each float)