Well, you need to have one that is, equipped with floating line, and a few that aren't, to mark where the MOB is and give them something to swim to. So we had a Lifesling, a big orange ring buoy and a two or three cushions. Don't forget to throw a life vest too.
Most importantly, you need to know how to use it, and that comes with practice.
"There's the Right Way, the Wrong Way, and what some guy says he's gotten away with"