Dutch nailed the level of difficulty of the attack. A few guys in a fishing boat, 100 yards of line, a large fishing magnet, GPS, etc etc.. So anyone could have done it.
I also expect that the damage is relatively easily repaired. If Putin did do it, he didn't pay much to create discord and distrust among us.
How does someone get a ton of explosives down to 300 feet in the middle of a sea in a row boat in heavy currents, set up the explosives so that the blast will be directed and concentrated to rupture the reinforced pipe and blow up a steel and concrete pipelines in three areas