My husband and I are around your age and we went through the certifications up to Advanced Open Water over a series of stays at Sandals Montego Bay. The dive team there is absolutely wonderful: the instruction is thorough and we felt safe and secure at all times. On one of my first open water dives the regulator (somehow - I dunno what happened there) came out of my mouth at 40' under - something I would have thought would have sent me into panic mode, but no, I had been trained so well I calmly retrieved it and popped it back in. All while my instructor was 3 feet away ready to pop it in for me if I freaked. I highly recommend this particular dive team - by now we've dived with all of them, and I have the utmost respect for their teaching skills in particular attention to different learning styles (my husband is a woo-hoo let's do it person while I am much more cautious about activities where you can't breathe if you don't know what you're doing - lol). Both of us are now passionate about diving, and are great dive buddies for each other, thanks to our primary instructor Big D :).
We've been snorkelling at both Sandals Royal Bahamian and Sandals Montego Bay - both places have decent reefs within 5 minute boat ride from the resort beach.
As for the vibe at Sandals - its all good in the ones we've visited. Almost always a range of people - young, old, in between - and a range of preferences - quiet, partiers, in between. We've been to a Sandals 6 times now and have never seen anyone get out of control or crude - alternately never felt like it was stuffy - just a relaxed good time vibe with lots of good eats and lots of things to do if you're inclined to move off your beach lounge chair. The Sandals people are awesome - excellent service all around.