Unfortunately, Atlantis will only allow non-guests and people without passes into the casino and that's it. You can't explore the grounds at all, and the grounds are expansive. To my knowledge, the walking tour is still the cheapest pass you can buy as a non-guest to just check out the property. The tour did provide roundtrip transportation. I don't believe there are shuttles unless you're on a tour, but a taxi would be quick and cheap there if you wanted to go to the casino. You can also visit the marina and surrounding shops for free as well.
If you're there from 10-4, you'd have enough time for the waterpark but I don't know if the value is there. It seems like a lot to pay for a fairly short amount of time and I would constantly be clock-watching, making sure we are packed up, changed, and back to the shuttle to get back in time. We also got off the ship somewhat late due to the hoards of people, but our time in port was 8-3. Might be easier to line up earlier to get off with a 10am port time.