Time to get wet.
Back in the '50s I built a lot of sail boats because the engine and fuel cost was zero. It did cost me some effort beyond building and rigging the hulls, though; my mother refused to make the sails after the first two or three, I had to learn how to run the sewing machine. A skill that has served me well in areas other than modelling since.
My first "Fancy" boat was a Sterling (?) model of the Chris-Craft 52' Catalina, LOA was about three feet, with a Pittman "Super Panther" geared motor, a Willard NT-6 wet cell battery, and an exotic (for the time) two channel radio. Flyball actuator for the rudder, proportional steering was controlled by pulse rate, forward speed by pulse on time. Or the other way around, not sure now. Battery life was very short, tube radios in those days. The boat now sits on my son's mantel, it hasn't been in the water for years.
About five years ago I got a wild hair, and decided to build another boat. Something for my grandson and I to play with, without having to go to a flying site. The lake was at the back end of the property. Convenient.
After I moved down here, one of the neighborhood kids knew I did RC, he came to me with a "WalMart" toy boat, wanted me to help him with it. Took my electric runabout off the shelf, and went with him to a local pond. He looked at his boat running less than 1 mph, and looked at mine skimming over the water, planing nicely, and actually bouncing the prop all the way out of the water going over swells, and decided his WalMart toy wasn't worth the time and money. Wanted a fast boat for himself. This finally gets us to airboats.
Made him a deal. Some used surface radio gear, a used Cox engine and prop, and enough wood to build an airboat. Fifty bucks.
I refused to build it for him, but I built one for myself to be a model for him to copy. He finished the hull, mounted the servos, but never went any further, (didn't pay me either) then his family moved away. I still have the one he was working on, still unfinished. But I did complete mine.
Design criteria included having the power loading such that the boat would have good speed, but not so fast as to be hard to control or tend to flip in a turn.
I made the hull more like a punt than a classic airboat, with a sharply raked bow and a shallow semi-Vee bottom. The sides are curved, and the outer hull is made from 0.016" aircraft plywood. The top deck is 1/8" balsa, with a hatch for access to the radio compartment. Except for the radio box, the hull is filled with NHP two-part expandable foam which both reinforces the skin of the hull and makes it impossible to sink the boat. Engine is a CoxTeeDee 05RC.
Small, but the design has worked out nicely.
Bill.