Many adventure races have a river swim as a discipline. Here is a video from the 2008 Untamed Virginia river swim demo:
Calleva's river swim will not have such white water, but you better learn to avoid underwater obstacles. Don't forget fins, they will help you to maneuver and move faster in flat water sections. You would be very wise to get a good waterproof case for your map; you will navigate while in the water. We recommend you to have a foam boogie board with a leash. You can buy one in most sporting goods stores like Sport Authority. Try to avoid the heavy plastic boards (aka ripboards), because you might run/hike with your boards for part of the course.
Don't forget to register before the end of the month. Registration fees will increase June 1st!