The European Commission has revised its battery regulations to exempt devices such as the Apple Watch from requirements that batteries be removable and replaceable by users.
I don’t think making the battery itself waterproof is necessarily the solution here though: you would need to make the battery user replaceable without sacrificing the waterproofing of the entire device.
There are ways of accomplishing this in devices though: action cameras generally have replaceable batteries with an extra waterproofing gasket for the battery tray. Those are even designed for frequent-ish swaps (though, outside of water).
I don’t think making the battery itself waterproof is necessarily the solution here though: you would need to make the battery user replaceable without sacrificing the waterproofing of the entire device.
There are ways of accomplishing this in devices though: action cameras generally have replaceable batteries with an extra waterproofing gasket for the battery tray. Those are even designed for frequent-ish swaps (though, outside of water).