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There is the Utter Inn in Sweden. From the most northern point of Germany it is 755 km
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There seems to be only one currently in operation: Jules Underwater Lodge actually is underwater and it is located in Key Largo, FL in the USA. You can read more about it on Wikipedia
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.
There are more concrete plans in Dubai to create another Underwater Hotel based on designs by a Deep Ocean Technology(I think) company out of Gdynia but again money seem to be be the issue.
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.