Some places that seem worth exploring using an underwater robot are:
The wreck of Wilhelm Gustloff. A German ship sunk by a Soviet submarine in 1945. Mass grave of 6000 people.
Underground lakes with the possibility of discovering new species or the skeleton of mammoth.
Gustloff rests at 45m.
Other interesting German wrecks in the Baltic Sea: Goya 78m
and Steuben 72m.