Also, how would Ukraine pull off such a mission in a sea it has no access to?

A boat, some underwater explosive charges, underwater ROVs, and a team to operate all the above seems likely to be well within Ukraine's capabilities.

The first underwater ROV I found via googling is $10K: https://www.deeptrekker.com/shop/products/dtg3#

It seems like something within the technical capabilities and resources of any functioning non-microstate.

And risk getting cut off from further military and economic support from allies? Highly unlikely.

How many people do you need to blast a pipeline?

Don't have to be in line with the government of a country.