'm not sure if they make waterproof cases yet, but I'd be worried about the saltwater deposits in certain areas, especially around the digital crown.
Again I doubt saltwater deposits would be any worse than with any other watch intended for deep sea diving, even around the crowns.
I will be on the lookout for a waterproof case because I'd like to be able to take it surfing and leave my phone at home. But there are a few obstacles to making a waterproof case, based on waterproof cases I've had for my iPhone:
1) it would need it's own watch band, as I doubt there's a way to waterproof it 100% with the Apple bands, which is fine, because I'd like to have a more secure band anyway for water activities.
2) It would be hard to allow the sensors to function on the bottom of the watch, but maybe not impossible.
3) It would be hard for the digital crown to operate, at least for scrolling and zooming, but it should be able to be depressed.
4) Force touch might not work.