4warmwishers,
From what I've read, Sapphire Beach is better for snorkeling (according to Fodor's). Fodor's says there is a reef on the right, or east, side near Pettyklip Point. You will also have views of St. John's as well as other islands from there.
Coki Beach, which is near Coral World Marine Park, also has snorkeling on both ends of the beach. Again, per Fodor's. Coki, however, is not a Fodor's pick like Sapphire or Meagan's.
The hype regarding Magens Bay is that it is one of the Top 10 beaches in the world and the water is supposed to be like glass. Per Fodor's, "the bottom is flat and sandy, so this is a place for sunning and swimmingrather than snorkiling". On the day we went last January, six foot waves

would hit the beach about every 5-10 minutes. One wave knocked my glasses off and DS from my arms. Luckily, it was my glasses I never found. Something else was way more important to grab quickly. Don't get me wrong, the beach is beautiful. We were just surprised with the rough seas. I guess a storm passed through the day before.
As for life vests, I'm not sure if
DCL lets you take them off the boat. What we have are the ones like you see at
Castaway Cay. They are yellow and have an air tube on the shoulder. Very compact without air and can be easily filled by blowing into them. We got them at our local scuba/snorkel shop.
Jeff