True, it's just water and no nutrition. They can have green or red leaf lettuce, romaine, and basically any of the other leaf lettuces. Just not iceburg.
There would be no reason to give a rabbit iceberg lettuce unless it was on a diet & it needed something to fill up the stomach a bit. Even then, I'd try only a bite a day for a few days, just to make sure it doesn't cause diarrhea.
Yes, iceberg, like what it's named for, is just a different source of water that the dieting rabbit might enjoy. And he or she won't even want to use salad dressing.