I’ve been involved in pipeline abandonment and decommissioning for 29 years.
You don’t dive to the bottom. You drop a big rare earth magnet on an aluminum rope cable (or if shallow like this — compared to the Gulf) just a nylon rope.
You connect and drop the demolition charge down along the cable.
That’s it.
It’s not hard.
“It’s not hard.” and quite inconspicuous too I bet.