Has anyone ever tried capping underwater?

i think head in a bucket of air to try and stop the shockwave in the water destroying your eardrums

What could possibly go wrong . . . ?

I doubt eardrums are the main problem - they're unlikely to rupture and  have a chance of healing. The real problem is risking permanent damage to the sensitive nerve endings in the cochlea (the organ of hearing in the inner ear). I'm told that both deafness and / or tinnitus are a real drag.

Or does the OP mean "from above water"?

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??

I think you better work remote to avoid problems.

Some friends of SC Cascade (Belgium) helped us with one of our projects in France. They enlarged the entrance of a spring (Vannon) underwater and by doing this the divers were able to enter a 355m sump and 700m dry cave. And it is not finished yet.

I'm not going to go too much in depth on the working methods on this forum

Some background about this ongoing project:  https://scavalon.blogspot.com/search/label/explo%20Haute%20Marne and on our FB page https://www.facebook.com/groups/scavalon/

If properly underwater (ie diving) strongly not recommended.  But, I think your main problem might be drilling the hole to put the caps in.

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.

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Drilling with pneumatic drill on a 300 bar cylinder or with a compressor

Tinnitus is sonething I wouldn't wish on my worst enemy, assuming I had one.
I've suffered for 15 years.

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.

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.

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That is exactly what we did. We called in some experts.

You could cap with underwater without a problem if the rock and cap were above water.

Likely something you only do once !