Replaceable batteries and waterproof ratings aren't mutually exclusive. Never have been. Ports are a bigger barrier, and those have been solved for quite a while. Hell, there are phones that are ip68 rated, with user swappable batteries. The samsung x cover pro is a decent phone overall that does it.
Think about all the watches with replaceable batteries that are waterproof, various cameras, rc submarines, etc.
It will require design changes, but there are already plenty of options to make it happen.
When I worked at Google I seriously had someone try to tell me that making a key fob waterproof was challenging. I never did figure out what that guy was thinking.