Here is a description of a simple Seaplane straight-float departure dolly.

 The dolly is very simple, using mostly angle iron with about 100 pounds load on the nose wheel. I also added a nose up angle to the dolly to help in lift off.
The nose wheel is my plane’s tail wheel with short cables to apply slight centering pressure.
The brakes are a screen door spring (visible on right side in last video) that rotates a 11 foot long pipe connecting a simple (over centering angle iron piece that bites into both rear wheels locking them up (evenly). The over centering is where the braking power comes from. It flat spots the tires but at a rate of once a year. 
The trigger for the brakes is an arm with a piece of rubber hose pinched under the float that is let free when I lift off.

Here is a video of a nice home built seaplane tug.