Let's see how much I can help you with. On Grand Cayman make a right off the tender pier and walk a short distance to the dive shop. There are steps going into the water.
Not really a beach. There are lockers, food and of course a dive shop here.
Almost anywhere on 7 mile beach is good snorkeling. I suggest you go with a group and someone be the designated "watcher" of your stuff, or leave valuables on board, or take an organized tour and they will watch your stuff.
Can't help with Roaaton.
Snorkel vest is an inflatable vest, worn deflated if you want to "dive". partially inflated if you want to surface snorkel and can use a little assistance and fully inflated to just surface snorkel or if you need "help". I highly suggest wearing one. The cost about $35
and pack flat when deflated.
Fins, if you are going to stay in the water a while , want to cover distances effortessly,
want to dive down at all, I highly recommend you swm with fins.(this stuff is usually supplied if you go on an organized tour, can be rented on some ships, can often be rented at full service beaches and dive shops.
Have a wonderful cruise