Recommended Min Temp for not using a wetsuit

My recommendation?  buy a wetsuit.

What's going to happen if you sign up for a race where you expect the water to be a cool, but palatable 68 degrees, and you show up on race day and it's 59 degrees?

Me- I'm a scrawny guy.  anything under 76 and I'm donning it.

I think it depends on the length of the swim and your tolerance for cold.  I've surfed in 50 degree water without a wetsuit, but it's very cold and takes your breath away.  I couldn't imagine swimming for very long in that temperature of water.  I'd say anything north of 60 degrees would be palatable (but very cold) for sprint swims and 70 degrees for Oly or higher.  I'm only guessing on the temperatures though so don't take this as gospel.  

OP asked what is the min temp for not using a wetsuit becuase he does not own or use a wetsuit....and instead of answering the question he asked, most people said "buy a wetsuit". 

Liked the OP, I don't own or use a wetsuit and have no intention of buying one.  The last race I did probably 99% of the people wore a wesuit and the water was like 77 degrees and felt awesome w/o a wetsuit.  Yes I was a little slower but I totally enjoyed the freedom of swimming w/o a wetsuit.

I live down south and don't do a lot a races and the races I do are in the summer so can't really offer up a good minimum temperature.  But I'm guessing it depends a lot on the distance.  I think for a sprint you can swim down to about 65 wo too much trouble...maybe even 60.  For an Oly and HIM, I'd say high 60s.

But this is just me.  It really depends on your tolerance to cold and how much you want to tough it out....and how fast you intend on swimming to generate your own heat.