You could make a prop harp from insulation foam, paint it, and coat it with a nontoxic waterproof sealant like silicone.
https://www.youtube.com/watch?v=4XucJ-Y1CUE
And it's possible to play string instruments underwater, aside from the whole rotting and being destroyed part. You could use a waterproof guitar, made from a material like carbon fiber, or more inexpensively, the
Makala Waterman is a plastic waterproof ukulele. I had one of these when I tried to learn ukulele a few months ago, and it was great. It's waterproof and borderline indestructible. I played it in the shower, though the fretboard cracked slightly (less than an inch) after I threw it about 30 feet and used it as a tennis racket to hit a banana.