Need Some Reassurance

1. You most likely will not be the only person there without a wet suit.

2. You'll be able to handle the water temp fine over that distance.  Try to get in before the race so you're not shocked once the race starts.

3. Everyone in a wet suit will think you're a tough guy becuase you can do the swim without one. 

You're a medium-sized dog with a rocket strapped to it's back. What could go wrong?

 You won't likely be the only one without a wetsuit. Get in a little practice beforehand so you can catch your breath. 500 meters is short, but it's a long way to go if you can't catch your breath (trust me on that one...). Good luck!

Everyone keeps saying you can handle it but your post doesnt say what the temperature of the water is anticipated to be.

If it is in the low 60's and up, yeah, you should be fine. Below 60 is COLD even at 500 meters. Really depends on you though as well. Some people can take it, some cant.

2 years ago I did a sprint, sans wet suit, in 62 degree water. I was pretty much fine after doing a short warm up swim and getting my body used to the water. Others were crying like babys while in a wetsuit.

You will make conversation though I am sure. people will ask if you are nuts. Just say yeah.