I have both stayed at the Atlantis and visited there off a cruise ship. There are a couple of ways to get there from the ship. You can take a water taxi from the area where the ships dock but you will still have a pretty good walk from where the water taxis dock to the Atlantis. Or, you can take a regular taxi which will take you to the door of the Atlantis. That's the best way.
You can not use the facilities unless you are a guest. They have security personel who will ask for I.D. to show you are a guest there. You can purchase a pass for $ 25.00 per person which will allow you to tour the hotel, grounds, the dig, etc. If you haven't been there, It's worth it to see the place. The passes are sold in the casino. Ask once inside and someone will direct you to the booth where they are sold.
If you try to sneak a swim or go on the slide, I don't think you would make it, but who knows. It's really a beautiful place to walk around and take photos, etc. The dig is especially interesting.