RE: Installing waterproff servo on the float
I just use fishing swivel-snaps on my water-rudder linkage. It's not necessary to cut the wires, just unhook them. I don't even use wires, just fishing line(Spiderwire), and 2-56 eyebolts for guides. Use 2-56 eyebolts and clevises at the ends to provide a nice loop for tying the fishing line.
If you mount the servo in a snug-fitting waterproof box or cavity in a foam plug, it will only take about 2 drops to fill the box all the way to the waterline. If the servo can leak, it will. I had my receiver in a snug cavity in an all-foam seaplane once. It took about a teaspoon of water in the fuselage to drown the receiver.
If you want to put the servo outside (ecch) why not just servo-tape it to the top of the float?