...but not swimming either (from the Apple website):
So triathlon or even general fitness swimming is out of the question.
Many triathlete don't use a watch during the swim in a race, because lets face it competitive swimming with a watch just sucks and your not going to look at it during the swim. Many, just put it on in the transition.
So, you get what resistance IPX7 rating of the Apple Watch gives you, the old Garmin Forerunner, which had the SAME IP RATING as the Apple Watch IPX7, was used in the swimming cap by this triathlete which as a long running blog on the triathlon (a decade).... He says he often used it without any protection there.
http://www.dcrainmaker.com/2007/09/how-to-swim-with-your-garmin-forerunner.html
But, generally, to protect his investment, he put the IPX7 rated watch in a ziplock bag under his cap to keep the GPS connection up as he swims in open water. Ingenious really.
Though, those watches are generally much cheaper, so losing them wouldn't be so much of an issue.
Myself, I train in all triathlon disciplines, but don't race.
I swim a lot and most don't use a watch alone (or at all) when they swim because that won't give them reliable heart rate info unless you've got an external sensor and it may be an hindrance in intense training (it is not improving your hydrodynamics for sure).
You can use various watches with a chest strap (like the Polar) if you really want reliable stroke rate and heart rate info. Another interesting device is a ear lobe and temple monitors that either tell you your rate directly transmitting sound through your skull or on your HUD/goggles.