A German student, that's who.
This is the Mercedes-Benz H20 concept, a submarine designed by Markus Krautter. Krautter is a student at Germany's Pfzorheim University studying transportation design. Submarines technically transport people, so it sort of makes sense that students would design them. Since Krautter is German it also makes sense that he would pick Mercedes as his inspiration. The sub seats four and has a unique shape that makes it look like a bottlenose dolphin.
Mercedes doesn't seem all that interested in making subs, but it's still cool to see an underwater Merc imagined.