Nothing jumpstarts technological development like a good war. WWII gave us UAVs, WWI introduced tanks, and the French-Indian War produced a unique gun battery — more cannon flotilla than warship — the radeau.
A radeau (French for "raft") is basically just that — a basic, flat-bottomed, multi-purpose, floating platform. They were especially popular as buoyant battle stations in the American Colonial era given their simple design and adaptability. But while all radeau are rafts, not all rafts are radeau. Vessels designated as radeau specifically are exceedingly rare, in fact. The last surviving (intact) example of this class of utility ship, the Land Tortoise, lies at the bottom of Lake George in New York.
Designed by Captain Samuel Cobb and constructed by British and colonial troops in 1758, the Land Tortoise was built to serve as a lake-based artillery platform in their fight against the French. The vessel measured 50 feet long by about 17 feet wide with room for seven cannons and would have been propelled by either a short mast or a set of 26 oarsmen, had it ever been fully outfitted.
The original plan was to build the platform then scuttle it for the winter, so that it would be protected from enemy detection by the lake's thick crust of ice. Come spring, it would then be raised from the depths, fully outfitted and set about ruining the weeks of enemy Quebecois manning Fort Carillon at the other end of the lake. Unfortunately, the workers loaded too much ballast onto the radeau, sinking it 200 feet down into the lake where they were unable to retrieve it.
The crew built a new radeau in the spring, the Invincible, and still managed to lose the fight at Fort Carillon despite outnumbering the French 5 to 1. The Land Tortoise, conversely, remained submerged for the next 232 years until a team of divers playing with a newfangled side-scan sonar system stumbled upon the historic find. In 1998, the site was designated a National Historic Landmark, only the sixth shipwreck in the nation to be bestowed the honour. The vessel is open for tours, just so long as you're PADI certified. [Neatorama - Wiki 1, 2 - National Park Service - Lake George Historical Organisation]