Maybe I missed this... but is this going to be a wetsuit legal tri?
If it is, then USE A WETSUIT! If it's cold enough for them to make wearing one legal, you probably need to wear it. Especially if you are not already doing OWS's on a regular basis.
At my first tri this year, the people who opted not to wear wetsuits were COLD! One girl just dropped out after she got into the water... another girl DNF'ed because she couldn't finish the swim because the water was too cold... and my friend got full on leg cramps because the water was so cold. He said he was a real dummy for NOT getting a wetsuit for the race.
Be prepared though, a wetsuit will give you the feeling of having your breathing restricted. I felt like I was being chocked the whole time. It was my worse swim ever... BTW, I hate swimming in wetsuits! HA!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"