I recently bought a Supernote Manta. It's an e-ink writing tablet. Guess what: it has a back which can be opened, and its innards are easily accessible. I could pop in an SD card, and the battery can be replaced, too. It's thin and light.

We are being gaslighted by Apple. They keep telling us that it's impossible to have a thin and light device with a user-replaceable battery, or even, heaven forbid, an SD card slot. I beg to differ: there are some compromises (it won't be as seamless perhaps and Jony Ive or whoever won't be able to wax poetic about the materials), but it can be done.

I would imagine something similar is true for waterproofing. There are certainly ways to have a separate battery and phone, with a waterproofed connector.