I'm doing the sprint DC tri and I don't have a wetsuit. I was a swimmer in high school and it's my strongest sport. 800 meters doesn't seem very far to me, but then again I've never done an open water swim in my life. Is a wetsuit really going to make that much of a difference? i'm a pretty competent swimmer....in the pool...
You sound like a stud in the water. For someone with your swim background, a wetsuit isn't necessary unless you are looking to podium within your age group. If you are just planning to be out there to compete against yourself and don't want to spend the money on a wetsuit, bypass the wetsuit. The only thing that should come into play in your decision making process is the water temperature. What is warm to some is cold to others. What is cold to others is warm to some.
There are plenty of people out there who do Ironman races without wetsuits. There are a few Ironman distance races where wetsuits are not even legal to wear. I remembered standing next to a gal at IMFL last year wearing just her bathing suit. My reaction upon seeing her was "damn! that girl must know how to swim" :)
It seems like if anything you'd want to get a john, not a fullsuit, being such a good swimmer (this is NOT something I know from experience, I swim like I'm trying to get a sweater off). Over 800 meters a wetsuit will definitely help you a bit, and remember that if you expend less energy on the swim you will have more energy for your less strong events. Might be worth renting a wetsuit from a local shop and trying it on an open water swim to see how it feels.
I'd like to add a bit of color to what Tuan said about "if you want to podium". There is a tendency in triathlon to buy buy buy whatever will make you faster. Will a wetsuit? Yes. So will race wheels, a better bike, an aero helmet, stop drinking beer (noooo!), etc etc. You can quickly sink a ton of money in this sport, and not saying that's a bad thing, but whenever you have to make a decision like this, it helps to assess what your true goals are (personally) in doing the race. In the end, this is fun, and do whatever is comfortable for you.
I'm a swimmer too and have done 3 mile open water swims without a wetsuit. My general rule is if the water is in the 50s (or below) or the swim is greater than 3 miles, I wear a wetsuit. For an 800 meter swim, don't bother - any benefit you gain by wearing one will be negated by the amount of time it takes to take it off, especially if you're not used to getting out of wetsuit.