Well, you're on antibiotics this week, so I guess the biggest risk is that you get exposed to something else and your immune system is already taxed.

Normally, swimming in better water, your immune system can take it. Make sure that you are well hydrated and up to snuff on your nutrition and your immune system can do the rest in most open water situations. Maybe after this experience take care to clean any open wounds very well after the race? I wouldn't let it keep me from racing, but I am super stubborn.