I bought a bunch of COTS RC electronics (A 2,4GHz transmitter receiver pair, an ESC, a few motors, and two batteries) and made them into a boat. I made the hull myself because I wanted a very specific type of hull. Contrary to most RC boats, I wanted mine to have a displacement hull so I could load it up with whatever cargo I wanted, regardless of weight. The top of the ship is attached with 6 bolts, allowing for devices to be bolted onto the deck if the need arises. I built the hull from fiberglass because it was very strong and was easy to adapt to any shape. Also, my boat can tow stuff around lakes because it has a motorized hook for attaching and releasing tow cables!
To decide upon a shape for the hull, I looked ships with displacement hulls and merged the elements from various hull types into something I thought would work well. I also decided that the ship's length overall would be about 1 meter. I drafted a scale drawing of the hull's profile from the side and from above, which I used as a guide when building the hull itself. I built the hull by laying fiberglass resin (which I bought from Home Depot) over fiberglass fabric, all of which was draped over a form. I made the form by gluing a heap of 2x4s together in a VERY rough hull shape. I then my 4.5" grinder to carve my wood block into a hull shape. I patched large holes with wall putty so the shape was generally smooth.