If you're talking about submerging an acoustic guitar and trying to get someone to play it underwater...well, good luck being able to play anything at all. Also, I doubt that you could even get the strings to resonate when surrounded by water.
i wonder what would happen if you recorded something above water and then record the same performance underwater and then use those to match eq the underwater source to sound like its not underwater. then possibly make underwater speakers that sound similar to normal speakers typically sound but underwater.