USAT, our sanctioning body, does not have a cold water policy, like ITU does. They leave the decision up to each sanctioned event.
However, Ironman has a cold water policy that considers water temperature, air temperature, and whether the water is fresh or salt. Ironman has altered swims due to cold water and/or rough conditions and will always consider environmental conditions and the safety of its participants. If necessary, a decision to alter the swim will be made one hour prior to race start and will be clearly communicated to the athletes.
Bottom line, with regard to St. George, race operations feels confident, based on historical data, that the water will be “warm” enough for the full swim distance.
For those of you doing any cold water swim event, here are a few tips I give my athletes:
Consider purchasing a neoprene cap to wear under your official race swim cap. (Don’t wait until the last minute, in the race town, to make your purchase.)
If possible, have a thermos of warm fluid to consume pre-race (tea, coffee, chicken soup, etc.) Pre-heating your core seems to keep people comfortable for a longer period.
Stay as warm as you can pre-race. (Keep your shoes on, sweatshirt, etc.)
Don’t “warm-up” pre-race in really cold conditions. Just take the first part of the swim as your warm-up.
If you haven’t practiced cold water swimming pre-event, know that the first time you put your face in the water, it feels like your breathing disappears. Know this is coming, relax and take a few strokes to settle into a rhythm.
An update for inquiring IM racers. I have not heard back from IM yet on the water temperature question. I don't know if they don't plan to have any guidelines, are working on it, have not seen my requests (to two sources now) or what. As soon as I know anything, I'll post it here.