Dolphin Inspired Mini-sub
What do you get the millionaire in your life who has everything? How about the Seabreacher mini-sub. Described as a dolphin-inspired cross between a jet ski and a submarine, the Seabreacher has a top speed of 45mph above the waves and 20mph below them. The two-man £30,000 craft is 15' long and its design makes it self-righting. Strangely, this doesn't come with a laser package.
My guess, reading between the lines and looking at the picture, is that the entire vessel is buoyant, and only stays submerged by moving forward and using its control surfaces to counteract its buoyancy. That would make this thing a "submarine" in the same way that a snorkeler is a "marine mammal": perhaps technically defensibly so, but not really what people mean. In any case, if that is true, then when the engine stalls, the vessel will simply float to the surface.
That's how I'd design something like this.
Making a "real" submarine takes a lot of engineering expertise. You can cheat any number of ways to make something sort of like a submarine. You can make a "wet" submarine, and avoid having to deal with a pressure hull. Unfortunately, that experience lacks something of the Captain Nemo drawing room atmosphere (namely the "atmospheric" part). Making a bouyant submarine that stays submerged using its control surfaces means you don't worry about ballast systems, and have a system that is intrinsically safer than one that requires pumping air and water around. But it means you can't go to deep, and you can't stop and admire the scenery.
Post may contain irony: discontinue use if experiencing mood swings, nausea or elevated blood pressure.
Because we can all think of how to spend a million, but the rich ones only think of how to make a million.