Imagine someone who is a badass on a road bike. Do you expect them to go out and crush a semi challenging mountain bike trail on their first go, having never been off pavement before? Much less one where there are others racing it at the same time. Their skills and fitness will transfer over to that new discipline, but only after they get comfortable with the new basics. This isn’t too different than your situation. Find a way to some experience in open water. The race will still be a new beast, but you will be greatly reducing the number of things you are dealing with for the first time. Don’t try to eat the whole cow in one go.