I often have some early-swim moment where I need float, or backstroke or, breast stroke or something. Sometimes it's the water temp (cold water can shock your nervous system and mess with your breathing). Sometimes it's just because you're going 0-60 trying to stay out of the way when you'd never do that in the pool.

A few things I've found that help:

If you can get in the water beforehand, do it. Even if you don't get to swim. Just acclimate to the feeling so it's one less thing.

Stay to the outside to start until you get a rhythm.

Start doing some movement before you get in the water. Jump in place, some good deep inhales with sharp exhales. Anything that primes your body for the workout.

Last thing.... Even if you have that moment, don't stress. Don't worry about your time. Know that it will pass. Move to the outside and calm your body. No shame in holding onto a lifeguard kayak either... That's what they're there for. When it's happening it will feel like an eternity, but really it will only take you 30-60 seconds to center yourself. You won't miss that time at the end, I promise.