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Long Drop said:If properly underwater (ie diving) strongly not recommended. But, I think your main problem might be drilling the hole to put the caps in.
Pitlamp said:Pneumatic drills are also very hazardous to hearing when used in water by a diver. We were first warned about this by none other than Dr Oliver Lloyd, circa 1980. It can be done but only with mitigation against hearing loss.
My advice would be, if you want to remove rock, whilst completely underwater, seek advice from someone who is experienced in such projects - and who is both a Cave Diving Group and a BCRA EUG member.