I assume you're talking about a land taxi vs water. The Atlantis is the biggest resort on Nassau and perhaps the Caribbean. Taxis are very easy to get from there and should only be about a ten minute drive back to the pier. I advise against taking a water taxi. I thought they were uncomfortable and took at least twice the amount of time than a land based taxi.

Nassau is pretty small and there isn't a heck of a lot to do on a one day excursion. Maybe a horse drawn tour and there is always the huge straw market. The aquarium at the Atlantis is very impressive and they have a large casino, by Caribbean standards.