I use this when I take my kids to the pool/beach, don't know how well it works while going caving.

https://www.amazon.com/Oterkin-Waterproof-Protector-Shockproof-Protective/dp/B08YNJG7MQ/ref=sr_1_3?dchild=1&keywords=s21+ultra+waterproof+case&qid=1623034079&sr=8-3

I test the case in the sink first just to make sure it's waterproof/sand proof before fulling using it.