For all the complaining that people do online about headphone audio jacks disappearing from phones, I've yet to see a real informed assessment of how waterproof it's possible to make headphone jacks these days. Meaning both sides of the connector.
Like, we all know there was not a secret conspiracy involved; phone makers shifted all their effort to making phones that didn't die when they got wet. Same with the batteries. So start there if you want to make a case.