Not to downplay the skill involved in lifeguarding, but the camera position/focal length in most of those videos offers an absolutely terrible view of those swimming compared to what the lifeguard can see.
In my experience, the challenge of lifeguarding is mostly a mental battle to stay focused. It's not that hard to spot people who are drowning if you're paying attention to what's going on, especially if you know what IDR actually looks like, as detailed in the OP. It's also pretty easy to pick out the people who are most likely to need to be saved just by watching them move in the water. But again, being able to pick up on that requires focus.
In my experience, the challenge of lifeguarding is mostly a mental battle to stay focused. It's not that hard to spot people who are drowning if you're paying attention to what's going on, especially if you know what IDR actually looks like, as detailed in the OP. It's also pretty easy to pick out the people who are most likely to need to be saved just by watching them move in the water. But again, being able to pick up on that requires focus.