There are any number of ways to rig the water rudder but one of the most popular and simpler ways is to attach a rudder horn on the opposite side of the other one run a flex cable or nyrod a bit forward and loop back to the rear and down just in front of one of the sterns (it makes no differance which float has the rudder). Asimple cable clamp to somewhere along the side of the fuse and one on the float ahead of the rudder tiller. Simple, easy and most important quick to change over to Landing gear but effective.
Contrary to popular belief you do not have to increase power for a float plane depending on what you want out of it of course its nice but certainly not neccessary in most cases. The real key to good water performance is the float selection and installation and a couple of key points are the relation of the float step to the aircraft center of gravity and the relationship of the wing incidence to the float incidence. Of course there are a number of other considerations too.
Do a search here on float installation and you will find a wealth of info.
John