Something the Apple watch could easily squash out of existence if they just squeezed an extra day out of the battery for the Apple watch 2. I would happily use a separate watch for my swimming metrics.
it seems to last more than 18 hours for many people, so unless you're planning to use it for sleep, you will likely be able to get two days by the time they reach third iteration.
as we all know, apple does not compromise design for battery life.
as it stands, the apple watch already features a pretty slow processor, no GPS, a power-efficient OLD screen that turns off unless it's being used, etc and it still stands at 12mm thick.
it's extremely unlikely that they're going to be able to add significant battery life to that by the 2nd generation while also adding more sensors and other battery-sucking hardware improvements.