What do you get the mariner who has everything? You could do worse than the Ego Compact Semi Submarine.
Made by South Korea's Raonhaje, the Ego Compact is more of a glass-bottom boat than a sub, but it's still pretty nifty. Protected by 20mm-thick acrylic windows, the waterproof cabin hangs down from two pontoon-like floats and serves as a mobile observation room.
I was on a glass-bottom boat recently, and thought it was the perfect thing if you're too lazy to learn how to dive. But if seas are choppy and boat is pitching, waves can prevent you from seeing well underwater. The Ego seems to get around that with its low-slung cabin.
The two-seater, 12-foot craft is powered by two battery-operated 1.5KW thrusters and has a top speed of 5 knots (about 5.7 mph). The batteries take 6 to 10 hours to charge, and will last about 8 hours at cruising speed. So don't expect to make it out to the Mariana Trench.
Still, it might be handy for fishing. One guy on deck, another in the cabin directing him to the fish. That, or shark-watching.
Released last year, the Ego has been making the rounds recently, but Raonhaje doesn't list a price for the half-sub. It's marketing the craft for leisure and research and says it's working on a four-seater version.
With that, the whole family can spend the day underwater.