When training for an open water swim, part of your training should be in an environment similar to the one you will be racing in.
If you are doing an ocean swim, don’t let the race be the first time you set foot in the ocean! It’s important to get your body used to the water temperature so it is not as much of a shock on race day.
Also, specifically with ocean swimming, getting used to waves, tides, salt water, and not being able to see to the bottom the whole time will pay off on race day!