Waterproofing a 3.5mm jack is not that difficult compared to a USB-C port. There are quite a few phones on the market right now which are waterproof and have a headphone jack: https://www.gsmarena.com/results.php3?chk35mm=selected&sAvailabilities=1&sIPCerts=11
The problem is that it takes away a chunk of space which usually means less battery life.