Yes, the fare is per person each way. There is no problem getting cabs, the problem comes waiting for them to fill up. They like to pile as many in as they can. They will be at the pier, downtown, and at the entrance to Megan's Bay and there will be tons of them
If you like snorkeling, they have equipment rentals and they have lockers for rent, too. They will have chairs and umbrellas for rent. There is a bar/restaurant area, but we usually go back to the ship to eat/change and go back into town. Be sure to have a drink at Paradise Point, I can't remember the name of it, but it's their specialty and yummy.
I buy some of those disposable underwater cameras at Wal-Mart to take with me. That way, if it were stolen (highly unlikely) I'm only out $9. They're 35mm, so the pictures are pretty decent. Beats losing a digital.