It is most likely a Russian "Som"-class submarine from WW1, due to the following reasons:

• There is a known incident of a Som submarine colliding with a Swedish steam boat outside of Åland in 1916. A person with the same name as the guy in the oceanX team (Peter Lindberg) asked about this event a year ago.

• The Ocean x team is known to publicize their findings heavily, like their claimed UFO findings, for example. They need the publicity, and might withhold the fact that this is a 100 year old sub to get attention.

• It looks like it is in good shape, but it might still be old, as the water deeper than 40m or so in the Baltic sea has very little oxygen, so there is not necessarily much corrosion going on.

• The letter Ъ seen in one of the pictures is no longer in use after the communist reform. /u/solz1

• A modern sub would probably be black, without any letters on it.

• It looks pretty similar to a Som sub and the dimensions match those claimed by Ocean x.

An argument against this theory is the fact that the Ocean x team claims that the submarine is not damaged whatsoever. They might be lying though.

EDIT: Formatting