While the ship may not enforce it outside of the main dining room, your fellow (non-Monkey) cruisers will be perfectly happy to give you the stink-eye if you’re That Guy wandering around in beachwear on the formal nights. Seen it happen. Have not been the recipient.
Cosplaying The Dude is better reserved for non-formal times. Yeah, it’s a real harsh on the mellow, but that’s cruising sometimes. 🙂