It's certainly not trivial to approach from below. The facility will be located 100 miles off the coast - it's not like you'll just jump in the harbor and swim

It's certainly not trivial to approach from below. The facility will be located 100 miles off the coast - it's not like you'll just jump in the harbor and swim that distance. So you'd need a ship to bring you close and then bring explosives with you. So you need them waterproofed and large enough to actually damage that beast. Puncturing a few holes in the hull will not significantly damage this. If you're trying to attack a LNG plant on land you can at least try and drive a large van through the gate.