It's the one ride DS doesn't want to ride anymore because it's so LOUD. Even though early morning is a good time for lines, we cannot ride it very first b/c DS isn't quite awake enough to deal with being "underwater" and having the noise of the sound effects and captain's voice.
At 4 he still dealt with it, but you just never know. And my guy is 8, and we are fine revealing secrets of rides to him. We had a long conversation about the ride building during our last trip, because he just insisted that it's all under water, etc. With a 4 year old you don't necessarily want to do that.
There IS a viewing area that doesn't involve the sub, and is just a room, where you can watch it, if someone has claustrophobic problems or an inability to get down into the subs, by the way.