You could try finding a municipal pool with a lazy river and swim the opposite direction. There are also practice swims from time to time down at the race site including the day before the race. Check with the various Facebook groups for details. Honestly, I don’t really think you need to practice swimming against a current. Most years the current is negligible or far more advantageous when swimming downstream than an actual detriment when swimming upstream. You won’t be swimming any differently from normal, just going a little slower against a current and a little faster with the current. If anything, you could practice increasing your arm turnover in a pool during a few sprint sets.