I would like to add my 0.02$:

I built several seaplanes in the past, especially pattern (F3A) models and slow flyers. I NEVER had to swap the engine to the next displacement to get airborne.

I also have to say that my models were also built very light and had good performance with wheels: no unlimited climbs, but enough power to keep it airborne and some extra power for when it is needed. Light construction is VITAL when flying off the water!!!

In my opinion, if the plane is already light, it all comes down to the weight of the floats: I build all my floats myself using high-quality, low weight isolation foam (white, 10 or 15 kg/m3) and sheet them with very low weight balsa and finally cover them with Oracover. On my [link=http://www.rcuniverse.com/forum/m_1123859/mpage_1/key_/anchor/tm.htm#1124170]latest 2m F3A pattern plane[/link], my floats' weight is 170g each (98cm length, 11cm wide - plenty of volume there).

Good, solid and light construction of plane and floats minimizes the need for a bigger engine. It helps getting on the step earlier and thus gain airspeed to take off from roughly any water surface. Of course, the added drag slows the plane down but should not be of any concern unless you need a lot of power e.g. to fly pattern. Just to stay aloft it ought to be enough.

Regards,
-Fabrizio