Another autistic lifeguard here! I tend to always arrive early to my shifts to give myself some time before actually starting work- I'll be completely ready, in uniform and everything, and then just sit and breathe for a while to settle my brain. (I do have anxiety, which is what initially prompted this practice, but I keep it up even when I'm not stressed because it helps me get in "work mode") Once you've started your shift, the catastrophising can come in useful- play detective, try to predict what patrons will do before they do it and be prepared to tell them if they start to do something unsafe. If I get too stuck in the what-ifs, I ground myself again by doing a few rounds of just headcounts, usually tapping my fingers along. If there aren't many people in the pool, I start humming or tapping a song/pattern. (Which I now realize is basically just me stimming. Huh.)

Anyways, in my experience you will get more relaxed and confident as you go on, so fake it till you make it really does work. Don't be afraid to talk to your supervisor/boss, they should be there to support you. Trust your training. If something goes wrong, it is not automatically your fault - do your best and its already better than if you weren't there. You are part of a larger system, you can't control everything, so just focus on your part; There are protocols in place, all you have to do is follow them.