That link shows a setup intended to be used above the surface. The hulls are nicely shaped for going through water easily with the camera looking along the surface while hoping that the very high centre of gravity will be compensated for by the spacing of the hulls. Mounting a large box under the boat will remove any performance efficiency and require a lot more power.
If hoping to see more than a few metres underwater, unless the water is crystal clear, a camera of the size and quality of the one looking at you on your laptop will do the job while sitting inside a much smaller housing.
To use the housing mentioned in post#1 will need, to coin a phrase, a much bigger boat.