I know what you mean about tan lines... The first cruise we went on I bought a beautiful spaghetti-strapped gown. The first day on the ship I ended up with awful tan lines that really looked tacky in the pictures we had taken on formal night. Nude sun-bathing (not for me ....), but would definitely bring a tube top next time.
PS ..there was a topless deck on the Carnival Sensation for those daring enough.