You can tell as you take the cap off if it was a tight seal there will be the pfffffft sound as the co2 escapes and if there is no rust on the inside of the cap it should be fine. Just taste it before swigging, only if it's been badly stored it might taste awful.
When the SS Politician sank off the coast of Eriskay in 1941, the islanders salvaged (and drank!) around 24,000 bottles of whisky from the wreck. So drinking booze that's been stored under water seems to be a safe enough thing to do ;-)
Treat it like vegetables and give them a right good scrub with antibacterial detergent.
Oh, and check each bottle for enormous worms. Can't be too safe.