View the swim course, including key turns, before your wave. Take mental note of the key turns and match them with something on the shore if possible. Also note on which turns you will have the sun in your eyes. If you can, swim the course, or sections of the course, the day before.
Once you start the swim, take slow methodical deep breaths. Avoid short, panicky breaths.
Do this: Look up at the sky, which has a calming effect, on the inhale. Then watch and feel the bubbles come out when you exhale. You can also wiggle your fingers lightly on the recovery part of the stroke to help trigger relaxation on your upper half. Also, try dorsi-flexing your ankles while swimming. It's an active stretch on your calves and something you can practice in the pool before the race.
Finally, a lot of people get vertigo when coming out of the water onto solid ground. At the end of your swim, stand up slowly putting your hands on your knees for stability. Pause, keep your eyes down to begin with, then bring eyes up to the horizon and start walking.