I usually tune out the requisite safety spiel delivered on tourist vessels; what, after all, could possibly go awry during an easy cruise in Caribbean waters? Today, though, I’m all ears, because this is no easy cruise: There are sharks involved. And I’ll be swimming with them. It’s a unique adventure that’s drawn me to Nassau, much to the chagrin of friends and family. “Go see them in an aquarium — or on TV. Do you really have to swim with them?” my best friend, Beth, moaned.