> has the means and maybe motivation to do so.

How hard is it to destroy a pipeline? Do you actually need an underwater UAV, or might lowering explosives on a rope and a timer from a small boat do the job?

Remember - water transmits compression waves really well, and the pipe is the only compressible thing around. The sea there is only 200 meters deep, so lowering it on a rope at slack tide and with a good GPS on the boat, I'd imagine you'd be able to get explosives landing within 10 meters or so of the pipe.