A month ago I did a 1500 yard solo OWS to get ready for a race. I failed to check the weather that morning because the day before it was pretty warm. That morning it was about 45* outside. The water was not much warmer than that & was probably around the 52* you will have. I still had my wetsuit, but it was pretty damn cold. I got in the water slowly (mistake) and started shivering. But once I started swimming, I forgot how cold I was & just swam. I did 3x500 and after the first 500, the temps were not bothering me at all.

Short answer, you will have a shock when you first get in, but get moving quickly and you will be fine.

I find there is a big difference for every degree below 58 or 59 DegF. So each degree lower feels like 5 or more. My biggest problem is my face, that vise-grip lock the cold has on it. Not pleasant!

After 60 DegF things are much better. I use a LS suit. I'll transition to sleeveless somewhere around 70DegF and then after than to my PZ3 swimskin.

I just jumped in for a very quick swim in the lake here, no wetsuit - I think it's around 61-62.

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Depends on the situation. If it's a race I will suck it up (remember one at 13C - there was a guy there who apparently didn't own a suit - I can't imagine how cold he would have been after the swim). If it's just a training session and there's a local pool I'll skip the OWS below about 15C.

Exactly what he said. Under 60 the water feels really cold and getting colder with each additional degree. High 50s is still swimmable but damn cold. I'd say the cut-off is mid 50s. Over 60 is fine. 65 feels the best to me. 70 is already too warm for long distance swimming.

I grew up swimming in the ocean in Northern California.
I don't own a swim wetsuit and don't use them in races.
I'll do open water races down to about 52-53ºF (~11ºC) without a wetsuit. Up to about 3 miles, more than an hour or so in that water gets to be a bit much.
That's pretty cold. Anything below 60ºF (~15ºC) is actually cold.
Above 65ºF (~18ºC) or so is nice, but doesn't happen all that often - even in SoCal.
70ºF (~21ºC) is great. Done a couple of races at 76-78ºF (~24-25ºC) in Hawaii and it was just too damn hot.