My only advice is find clear water on the swim and pace yourself .. open water in crowds can be taxing mentally and physically. Trying to survive sucks compared to just swimming along .
I don't trust my knee for 10k (yet) so I'll be the one taking forever in T2 putting on a knee brace. ;) see you in the sprint

thanks. I'm doing the Sprint distance. hoping to find a shady spot for the swim and try to resist the urge to push too hard and just enjoy it. have a good race!

That anecdote has zero applicability here. That happened at the "Florida Phase". By the time the kids hit that phase, they are pretty much shattered. They've lost 10-20lbs because they've not rec'd enough chow, they're incredibly sleep deprived routinely going 2-4days with not the slightest catnap, and they've been on the move almost non-stop carrying a load easily >100lbs. They could move thru the cold water for hours, not generating much heat because of the slow pace of the movement. The FL phase is really tough and making it day by day only only grit and meanness. When one is already low in energy reserves, cold, wind, rain, all those "exposure" pieces are really tough on the body. Nothing like a rested athlete going hard for 15-30min.

http://gress.org/...ger/RangerSchool.htm
About 3/4 of the way thru, there is a section titled "Tim Parks, Ranger Buddy". That's a FL phase story.

'85-'88 I was on the college triathlon team in San Diego. We used to swim off of La Jolla Spring -> late Fall. Our temp limit was 63deg. We were all broke college kids, most of us didn't even own a wetsuit. 64-68 was entirely do-able. We'd wear double swim caps, those we could afford. Prior to jumping in the water, we'd go for a jog and do some calisthenics to both reduce the need for a swim warm up and also to elevate body temp. This was the Speedo era, so it was a little goofy by today's standards. Then we'd hit the water and immediately start swimming hard. Hitting the water was a shock at first, but it only took a couple seconds to get used to it.

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Books @ Amazon
"If only he had used his genius for niceness, instead of Evil." M. Smart

Pretty much this.
Below 24c and i start to struggle, I'm around 61kg and pretty lean and vascular. Did a pool swim in 21.5c and was shivering after 2k, where other people love it, more so the heavier guys.

I melt in that kind of water. Anything over 80 is too hot.

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I swim in the ocean regularly and 19C is not cold especially if I'm swimming and not just wading in the water. Below 18C is difficult for me though hubby who has more insulation doesn't have any problems.

It's dependent on the person as everyone says. The initial dip is always a shock to the system but depends on how well and fast you can warm up and stay warm.

The "heated" pool I swim in is usually 20-22C, for me the ideal temp for lap swimming.

It's highly personal as many others have noted.
I'm pretty comfortable at 19 degrees but then I find typical pool temperatures unpleasantly warm.
There are a few members of my tri club who did an open water swim with me last weekend without their wetsuits in the Irish sea just south of Dublin. The water was around 14.5C I believe (58F). That's a bit cold for me but some people are fine with it as long as they don't stay in too long or hang about when not swimming. They did about 1500m last week. Heck some nutters swim wetsuitless when it's under 9C (48F) but I think they generally keep those swims very short - with the exception of some "ice swimmers" who swim a mile in water that's only barely above freezing. Of course that's an event that requires a doctor on standby to check swimmers as they warm-up afterwards (that's the most dangerous time as blood get's circulating again and core temperature can plummet).

For training I would recommend train prober sighting in the pool. There are some easy drills and techniques you can do basically everywhere.
On raceday It really depends for me. Everything longer than a sprint I would get a wetsuit for sure! For a sprint distance it depends on a lot of factors for me, air temperature, type of swim (is it in the sea, a lake, canal etc.), is there probably a small running portion or a second loop, how far is the transition area from the swim exit? All those factors have to be considered. Eg a local sprint distance with a 500m swim in a lake, which is 250m swim, 75m worth of running at the beach, followed by a second loop of swimming. The running portion you will be way faster without a wetsuit, and won't heat up that much. The way to the transition area is rather short with 200m of running, so there is not much time to prepare your wetsuit for changing while running. Also water temperature will most likely be around 20°C and Air around 25-30°C so quiet hot. In wetsuits i tend to heat up really fast, so for me the given example would be now wetsuit. The tight course and the running will save up, the 5 seconds I will lose as a good swimmer on that swim leg without a wetsuit. Most important for me is, if the wetsuit -> bike transition takes 3 seconds longer than planned, the front pack is gone and i have to make up lots of ground on the bike (draft legal tho). On the short distance it is just not worth the potential extra trouble you could get in.
For a long(er) training swim I would always choose a wetsuit! Even in outdoor pools I often use mine, as during this time of the saison my body gets too cold after 4k of swimming at normal pool temperatures, as I am at full race mode :D

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sent from my iPhone

I swam at a local beach today, 17°C air, 19°C water, very windy and choppy and with little rain. I didn't use a wetsuit and swam for an hour, and actually feel this is the perfect temperature and environment that I can swim for hours if I'm not tired.

I also swam at the same place yesterday afternoon, with 20°C air, 19°C water, windy and choppy as today but no rain, for around an hour. I met a local tri club doing their OW training, however there was a guy not having a wetsuit got hypothermia and had to be pulled.

A 14 year old zombie thread, love it!! And I bet half speed now has a new answer than he had back then, he swims in low 60's/ high50's just about every day now..

The one thing I didn't see mentioned, is that 66 in fresh water is nothing like 66 in the ocean, night and day difference. 66 in the ocean is pretty nice for most, 66 in a lake is not really nice for anyone, except a few polar bears...