Dion,
to keep the prop out of the spray from the floats, put the bottom of the prop arc around 1 1/2" higher than the top of the floats.

The rule of thumb for float spacing is 25% of wingspan.

the step of the float should be under, or slightly behind the balance point of the plane. Also, there needs to be some float sticking out in front of the prop, both for stability and to keep you from striking the prop on stuff when taxiing.

The floats should be lower in front than in back. Another way to say it is with the top of the floats level, the wing should point upwards a bit with a Positive angle of attack of 3-4 degrees.

If you do rig up a water rudder, don't make it stick down very far. It is used for taxiing. You want it well clear of the water when you're up on step trying to take off. If the Water rudder digs in, it'll cause enough drag to keep you from hitting takeoff speed.