I think the "free vest" deal is only for the snorkel lagoon where they keep a stack of them an stop anyone going in the lagoon without it. Don't know if they would let you take it to Serinity Bay or not (if that's where you planned on doing any snorkelling).
They are expensive, but we decided to spend the money to just get some our selves. They have those values where you blow into it to inflate it, so just like the snorkel, we prefer to have something others have not had their months on.