Warm Up to the Conditions
This is especially useful in races. The last thing you want to do is get into the water and realise that the water is colder than what you thought it would be, or the conditions are chopier. You don't need to spend a huge amount of time, but even five or ten minutes, or swim out to the first turning buoy to get yourself feeling a lot more comfortable with what you are about to take on.
Generate Confidence in the Water
Sometimes you may need to get back to basics to build your confidence in the water again. For me starting with the basic Total Immersion Drills (https://www.youtube.com/watch?v=qMT_nUv8O4s) helped me get my head under the water and make myself more confident with what I was doing.
Wear a Wetsuit in Open Water
A wetsuit will keep you warm in colder conditions, and give you some extra buoyancy for safety. If you ever feel anxious, out of breath or confused, a wetsuit will give you the extra buoyancy to allow you to float with little effort and allow you to compose yourself again. Keep in mind that if there are other simmers around you may need to get out of there way first.
I guess the first thing you need to do is work out where your fear actually stems from. For me personally, I think it was mostly the deep water and a fear of the unknown.
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