Not sure where you'll be on vacation but it seems likely there'd be diving involved?
One of our granddaughters started learning snorkeling at Buddy's in Bonaire at age 6. She's now 13 and completed her 100th dive in Fiji last June.
I'd endorse Buddy's program 100%, plus the parents get to dive 1/2 day while the kid(s) are at Buddy Ranger camp.
My perspective... (No compensation - just a pleased grandfather)