It may come true in the future that cordless power tools will be waterproof but the current crop are not.
But with brushless motors now appearing it may be possible to have waterproof tools at a reasonable price,
or should I say affordable price for the average user.
For underwater work I used hydraulic or pneumatic driven tools. Depending on where you are the hydraulic tools can
be a problem if they develop a leak. Then you might have a reportable 'spill' on your hands and have to deal with the
cleanup costs and making reports to USCG, EPA, and others. We have a pneumatic Hilti drill on one job for wet work
which was not cheap, but neither are the electric Hilti drills as we all know.
Toughest tool to find for underwater work has been torque wrenches. They're available just not from too many sources.
Seem to be used mainly on oil platforms and by commercial divers.