Don't fall for the 'glue cuz of protection' myth - there are and had been water-resistant phones way before Apple started glueing to avoid customers doing their own repairs and them losing out on new sales.

Third parties offer new iphone batteries so it's clearly replaceable commercially

Conformal coating is a little more expensive than gasketing, but it works much, much better under pressure. Motorola does this.

Putting the battery outside the water insulation zone might work for that, it's a sealed pouch anyway.