I worked for RCI for four years, and I have a little secret for all of you...
There is no international law against topless sunbathing. Period. Deck or no deck, if you decide to sunbathe topless on an RCI ship, no one is supposed to bother you. I worked with the kids' program there, and we had to stop taking the kids across the top deck because every so often (rarely, but often enough to be a concern to certain parents) we would pass by some topless lady and have to deal with all these kids oohing and aahing over the "naked woman". Not fun, so we spoke to the Hotel Director about what we could do and found out...nothing.
So the next time you're on RCI and feel like whipping off the top (not the bottom, bottomless sunbathing is a violation of USPH rules), go ahead and do it.