So my question is, what kind of things will he notice or experience that are different than swimming in a pool? What's it like, to swim in the ocean? I can barely swim in a pool myself, and though I love the ocean and the beach, I've never been any deeper than chest high because I can't swim well. I've googled "swimming in the ocean vs. swimming in a pool", "comparison swimming ocean and pool" "experiences open water swimming" and various other keywords and phrases, and none of these articles and blog posts are giving me anything useful... it seems to be mostly about actual athletes training for triathalons or the like, not just a regular kid's first experience at the beach.
He's not alone, btw... he has a friend there with him who is an expert swimmer, but he hasn't been to the ocean since he was a kid. He won't feel confident enough to go very far from shore, but they'll probably be farther out than I've ever been. There won't be a lifeguard, and they'll probably be there in the mid to late afternoon. He's probably around 5'9" and um... pretty slender build, if these things have to be taken into account...
I'm probably rambling. Please let me know if you need clarification, it's late and I should have been in bed already. I also don't really know what to tag this, so feel free to add anything!
edit: I've gotten so many great answers! Thank you guys so much, I think this is going to be a really fun scene to write!