For water rudder, I simply cut a piece out of an old soup can and solder it on the end of a piece of music wire. I had to run another pushrod off the rudder servo for control, but I wanted a steerable tailwheel anyway. I just stick the music wire in the tailwheel bracket.
As for floats, they work great. I've used the Cunningham float design that was published in RCM for 5 sets of floats (5 different planes), and they all have worked flawlessly. The plane hops right onto step and flies away. With this float design, as long as the step is right under to 1/2" behind the CG, and there's a little positive incidince btw the wing and the floats they work great. I just use the "that looks about right" method.
Here's a link to the cunningham plan:
http://www.flyinglindy.homestead.com/skisandfloats.html
Later,
Matt