Capping is dodgy enough of the surface, but through the methodical use of various techniques, has, these days, been rendered 'reasonably' safe.
If you mean properly underwater, then you add veritable boat load of issues that are going to increase the risk factors considerably. Not saying it cant be done, but you're going to have to work through all the risks and come up with techniques to mitigate them. Not least of which is ear drum damage. Good Luck with that.
If you mean just under water, but having the rod above water, then yes, we've done it. Mixed success rate. we were capping boulders in a stream bed, too big to lift out of the water. Some caps fired no bother, others didn't. I suspect this was part luck, but thinking back, it was probably those that didn't go first time, that subsequently failed to go at all. Probably because they got squashed first time, and then wet. Little bit of candle wax in the crimped end might help water from getting in??