we did captain bryan's on the 5/24 magic cruise and were happy we did, as
DCL cancelled their excursion that day due to choppy water. capt. bryan's still went out in spite of the choppy water. it was a bit tough swimming in it, and i unfortunately couldn't take my 4yo over to the sand bar to see the stingray as a result, but it was still an incredible experience. i got to hold one of the stingray...WOW! i highly recommend doing this excursion (either with disney or on your own).
about capt. bryan's...one of their boats (the Hannibal) is a triamaran, meaning it has three pontoons (is that the right word?). we did the Golden Eagle in St. Maarten on last year's cruise and wanted to go on a catamaran again this year, so we specifically selected capt. bryan's as they appear to be the only ones that have such an option. however, unlike the Golden Eagle, the webbing on the Hannibal was close-knit and did not cover the whole front end of the boat as on the golden eagle. on the way back, there were five 10-15 yo kids lying side-by-side and they took up the entirety of the webbing (on our side of the boat). the rest of the front end of the boat is solid.
on the issue of going outside disney's excursions and doing one on your own, we started doing this last year when our then 3yo would have been excluded because of disney's age limitations. and, in many cases, they are disney's limitations. the providers we've gone to directly do not impose the age restrictions that disney does. so, last year, while disney would not let us take our then 3yo on the golden eagle, the golden eagle folks had no problem. we signed the usual waiver for him as we did for our older boys.
the second advantage to not using disney (for some excursions) is that the cost is oftentimes lower if you book with the provider directly. we found that with capt. bryan's stingray city in grand cayman this year and with the golden eagle and atlantis last year in st. maarten and st. thomas respectively. however, we booked the dolphin experience with dcl this year because there are so many cruise ships in cozumel on wednesdays that the provider doesn't really take many reservations directly. they are too busy with the cruise excursions. that's for the experience, not the swim.