I think you need to listen to the rules, for example at POC, they tell you not use a flash. There are actually many reasons behind this, including the fact that using a flash may reveal how some effects work, which they don't want.
On it's a small world, it is ok to use a flash, if it will not disturb others, i.e. you are the only party on your boat and the boat in front of you and behind you are empty.
In my opinion, outdoor attractions are fine, as long as it is safe to do so. The Jungle Cruise skippers for example don't mind at all. That being said, Flash pictures don't come out very well, unless you have an external flash, due to how far away everything is from you. Remember most camera internal flash's are good for 5-10 feet, at most.
Bottom line is if you are in doubt, either don't do it or ask a CM.