Most places that do snorkel outtings including the boats give you those little blow up kinda things. I can only describe the way they fit me as the blasted thing came up around my neck. It goes over your head and straps around your waist and between your legs....sound, comfortable...no !
I don't think you need to buy one or rent one if you do a snorkle trip with any one.
I would bring one if I had a child with me (I am not sure if they provide life jackets for children on snorkling trips, but I would feel safer with my own). That being said it is unnecessary to bring one if you are an adult (I have been on a few snorkling trips and they all had them).
One other point worth noting is the fact that the water can be a little rough by the Antilla shipwreck (at least it was when I was there), hence, if you are not a strong swimmer, you may want to wear the life jacket while snorkling around it.