The issue of hearing damage with power tools underwater has been well documented in CDG literature over the years (starting in Oliver Lloyd's era). Long before the vibrations become intense enough to affect the eardrum itself there is a great risk of damaging the auditory nerve endings in the cochlea (hearing organ) of the inner ear. This damage would be permanent. My advice is not to mess with power tools underwater unles you've thoroughly reasearched the hazards and mitigated against them in such a way as to eliminate risk. You only get one set of organs of hearing and when they're gone they're gone. And tinitus is such a drag . . .
I think the problem with electric drills is the issue of incompressibility of water in the percussion mechanism which can adversely affect efficiency. If you really must try this, try it with someone else's drill!
Frankly, banging in an 8 mm anchor with a small hammer is by far the easiest option for your situation, as Gareth has pointed out.
(Readers might like to note that I've now been in discussion with irnbru directly about this; I'm really posting these thoughts here to emphasise the problems of certain options discussed above, in case searches by others in future pull this topic up.)