TL;DR: BEST MONEY SPENT ON VACATION. Great time. Do this excursion. Wear sunscreen.
This experience was, hands down, the best part of our vacation to the DR. We stayed at the Hard Rock, and pickup from there was quick and stress-free. Don Julio was our guide for the day, and he and the crew did an incredible job making the excursion safe, fun, and entertaining! Of my group of 5 friends, we each were impressed by how it seemed like we did SO much and were out enjoying ourselves for SO long for only a half-day excursion!
The snorkeling was great (my first time), both in the open ocean and in the natural pools, and the drinks on the private beach were great! Hands down the best part was seeing the artificial reefs in the natural pools--the water was waist deep, clear, and mostly calm, and the reefs were full of life that was so cool to see up close. The second-best (possibly the absolute best) part of the excursion was the meals! My group of friends tried the fish, meat, surf and turf, and seafood meals and we all shared; every SINGLE thing was delicious (the fish was THE BEST I HAD on our entire vacation).
The drinks and music were also great, and there were even snacks between the first and second destinations on the boat ride. The 20 or so people on our boat weren't too crazy, everyone was just having a good time--though that likely depends on who happens to be in your group.
There was also a very brief time for shopping at some touristy stores at the end of the trip, which were great to grab some souvenirs for the trip at a less-than-resort-cost. You couldn't spend your money in a better way on your vacation to the Dominican Republic!