it turns out you're only supposed to use "swim mode" on the Garmin 310xt if you wear it on your wrist. If you put it in your cap with swim mode on, the distances are a pack of lies. Eyeballing the map, what the Garmin thinks is 0.10 mile is more like 0.17 or 0.18 mi.
So my AW is that my ass-slow OWS times were just wrong, and I think I'll be at something in the 15-19:00 or even faster for the swim. That's still a big range, I need to get one more OWS in to see where I am! Also it means that my OWS endurance is almost already where it needs to be for the Oly tri in September!
Unfortunately, the 310xt is just shitty when it comes to tracking open water distance. If it's on your wrist, you lose the signal every time you put your arm under water. I only use it to track time on ows.