6 hours is not a lot for sailing / swimming / snorkeling and lunch. You’ll be alternating activities between sailing and taking swim breaks so it’ll past fast.

Sounds too rushed for me, if doing 2 spots.

No, it looks amazing!
As others mentioned, just make sure you are protected from sun, wind chill, and rain.

I think you're underestimating how long it takes to travel distances via boat/water.

One hour to get out to the first snorkeling spot. One hour there. One hour to the next spot. One hour there. Beach time. Lunch time.

One-two hours sailing back to starting spot. It will go quite quickly.

We charter excursions like that when we do Caribbean vacations. They are always highlights of the trips. Bring a book if you’re worried you’ll be bored, but I promise you it’s 100x better than wandering through duty free shops in the port.

What is your alternative? Hang out on the big cruise ship all day? the excursion sounds like fun and 6 hours is nothing.

Take good uv shirts