You don’t just swim down 300 feet with a snorkle and a grenade. They don’t have the tech/capability dumbA !
Many divers have the training to descend 300 feet with the right equipment and gas mixture. I would imagine that many of those divers also have training in explosives.
Probably used this
https://en.wikipedia.org/wiki/Surface-supplied_diving_equipment
The story implies that, as they mention a decompression stage in the dive.
I know a guy who used to do this.
Ukraine has a lot of commercial divers it turns out, who do indeed work on gas rigs and gas pipelines around the world.
Probably learned on the considerable gas field west of Crimea that was active till 2014. There are dozens of rigs there.