When we were there, you had to have the wrist band to go outside where the pools or beach are located. They had someone standing at each doorway making sure.
Do you mind me asking when this was? I ask because during my last sailing March 2014 we did pay for the Aquaventure so we were able to walk around freely however, we were never saw anyone watching for wristbands which was a good thing since both DS and DH lost theirs shortly after we arrived at Atlantis.
The security guards that check wristbands are at the entry points for the outdoor grounds and water attractions. Once you're inside that perimeter, some staff might occasionally ask to see a room key or a wrist band, but not often...they rely on the guard at the entry points. Some hotel guests skip getting wrist bands altogether and just show their room key.
No walking around the pool and aqua venture area, nor the aquarium and sea life lagoons, unless you purchase a day pass or do an aquarium tour. But you can walk outdoors around the marina village shopping area.
How much is a day pass?
I don't think you can access the Cave Grill unless you come off the beach. There usually a guard checking for room keys or wrist bands at the Beach Tower exit. Plus, Cave Grill is often closed a few days a week, and closed for extended periods when the lazy River and river Pool by the beach tower is closed (which it is now, reportedly).
You don't need any pass to go in the casino. The Coral Towers and Beach Tower interiors and the casino are open to anyone any time. However, the Coral and Beach towers are old hotels that predate the building of the main Atlantis complex, and have just been repainted and redecorated to fit the theme a little better. Almost all of the coolest architecture and art is in and around the Royal Towers, which requires a wristband (or a room reservation) to enter during the day.
