I'm trying to find an underwater hotel that is not too far away from Germany. What's is the nearest one?
The resort that @RoflcopterException mentioned in the comments Poseidon Resorts never appeared to have gotten off the ground and would have been located in Fiji.
Atlantis Dubai offers Underwater Suites though they are not actually under water. Just appear to be that way from inside.
P.S. If you feel like sifting through the rumor mill there is a lot of chatter about various underwater hotels.