Airhead: THere's a lot of merit to what Dogdirt says. I am also reveling with pleasure at this point that I use my actual real name for a screen name. But I digress.

Your first water-plane depends a lot on your experience.
Most any trainer works well with a PROPERLY SELECTED AND INSTALLED set of floats. A Senior Kadet on 40" floats will give you face cramps from grinning. It's an absolute ball. I fly a Senior Telemaster on 45" Floats, and it's fabulous.

If you are an experienced flyer, then slap a set of floats on your favorite sport plane. AstroHogs, Sticks, and Morris Profile Planes all work well on floats. ANy of these are convertible: You can bolt the wheels back on, and go flying from land.

There are some good seaplane kits out there. The Ace Seamaster leads the pack, and flies like a pattern plane, but is currently only available as a .40ARF. My only criticism of it is that it has a flat bottom, and it tends to skip and slide a bit on landing. BalsaUSA offers several (Northstar, Laker) The northstar is a delta, fast, handles rough water well, and reported to be a pain to build due to a lot of parts at funny angles. Also, you are well advised to reinforce the bottom. The Laker is a nice gentle sportplane with almost trainerlike characteristics.

IMHO, the best seaplane ever was the Hanger Designs SeaCruiser. Recently discontinued. Grab one if you can find it, or get the plans and scratch it.

I have been designing my own seaplane. Once I'm happy with it, I will offer the plans. It's the Dover Darlin'. (Our club pond is in Dover, FL) Symmetrical airfoil, v-bottom hull with spray deflectors, and this week it has a v-tail to get the tailfins away from the water without the fragility of a T-tail. It has an OS.46LA now, thinking if it flies well with this, it'll really go with a "real" motor. The OS is beautifully friendly and reliable, but not very powerful.