Hey guys.... If this is your first Atlantis Cruise... you're gonna have a great time.
The theme parties are actually alot of fun. The first cruise I was on I wasn't really aware of the parties and didn't have much in the way of costumes. For some of the parties Atlantis often has an area where they sell items that people can wear to the various parties.
Some of the theme parties are the same most trips so that people know what to bring. Some of the costumes that people come up with are really amazing and alot of fun.
Some of the "staple" theme parties are the Dog Tag's Tea Dance (military theme), Disco Tea Dance, White Party etc. If you've already seen your Pre-cruise documents you'll know already what Theme parties will be held on your cruise.
To answer your question directly... if you don't dress up you probably won't be out of place... there's usually lots of people that are onlookers, but if you do get into it, it can be a alot of fun... My advise... bring some costume things for a couple of the theme parties... you don't have to go or participate in all of them.
Have a great trip guyz.... im envious of you...
Ian