My sister was a lifeguard there, and she told me it was really stressful. You have to go through an extensive training session (although you might not have to since you're already trained). Plus, there are tests throughout your shift. For example, people from the testing facility will come and do things to test you while you're on your shift. I had one friend tell me that they would throw the outline of a body on the pool floor, and you had to find it in a specific amount of time. Disney really cares about their guests, which is why it's really stressful. But, the other lifeguards i've met have been fine with all of it. I guess it's how well you can handle it! Hope this helps!
