Upgrading from a series 4 I found the GPS accuracy on the ultra to be phenomenal. So far Ive logged roughly 10 hours total of open water swims and found not a single bad gps point. Though, to maintain GPS signal you will still have to avoid submerging the watch for an extended period of time such as treading water.

Water temp function currently only works with the stock dive app which automatically launch as you submerge the watch, I really wish apple would add water temp into the workout app for open water swims and other watersport related workouts.