My boys both have HFA and they both had no issues waiting close to 30 minutes to ride the Aquaduck a couple of times. For one, you can see the jumbo screen playing movies while you are waiting outside, so that kept them focused. Then, once you get inside the funnel, it's "almost our turn" the whole time, so they did fine. Once they ride it for the first time, it is a LOT easier for them to wait the next time because they know how awesome it is, and it's worth the wait. 
On our cruise last August on the Dream, they were EXTREMELY strict about making sure that everyone who got in line had on NO SHOES, NO HATS, NO SUNGLASSES and NO CAMERAS. We waited in line barefoot all 5 days, and the stairs were never hot at all, and they had some sort of carpet/turf like material in the line area prior to the stairs. We joked around that they used "magical" paint on the stairs because they weren't hot. Actually, we realized the little yellow duck foot prints on the stairs ARE magical...they are made of a paint that does not retain heat. So, just stand on the duck feet while you are on the stairs and your feet will be fine!

On our cruise last August on the Dream, they were EXTREMELY strict about making sure that everyone who got in line had on NO SHOES, NO HATS, NO SUNGLASSES and NO CAMERAS. We waited in line barefoot all 5 days, and the stairs were never hot at all, and they had some sort of carpet/turf like material in the line area prior to the stairs. We joked around that they used "magical" paint on the stairs because they weren't hot. Actually, we realized the little yellow duck foot prints on the stairs ARE magical...they are made of a paint that does not retain heat. So, just stand on the duck feet while you are on the stairs and your feet will be fine!