It takes a country with spec ops divers... So Poland could do it, with these:
Breaking water with an HK MP5 mounted with an EO-Tech holographic sight is a Formoza operator Since the end of
www.tactical-life.com
Could be Russia, but that would be a pointless effort for them - if they want to cut off the gas flow, they have more reversible ways on their end of the pipeline, and they can use them as a bargaining chip, whose very existence creates a temptation in Germany to be soft on Russia in the current crisis.
Could be UK or USA, for the abovementioned reason - to ensure that Germany rids itself of temptations to do that.
At least one Polish politician leans towards that theory:
Could have been one of the Nordic or Baltic countries, Sweden, Finland and Lithuania have navy divers for sure, dunno about the rest.
Basically any country that can get a navy diver unit into the Baltic and doesn't like Germany wavering on Russia deals could have done it.
And that's enough countries for plausible deniability to whoever did it.
The scale of this effect is waaay too small for that to be the goal.