There are 2 staircases to that deck (at least on the Ecstasy) with signs posted.
Something like adults only, tops optional. It's been a few years. My son & nephew found it on our first cruise in '91. They were exploring the ship & they didn't see the sign. Dad & grandpa had to check it out when they heard there were girls up there with no tops on. We then told them they were no longer allowed up there. ( They really didn't see the sign, but we then pointed it out to them.) I don't know if they do it differently now.