I worked as a lifeguard at a lake's swimming area as a teenager, and I think it's a little less difficult than it sounds. It's not like staring out and hoping y

I worked as a lifeguard at a lake's swimming area as a teenager, and I think it's a little less difficult than it sounds. It's not like staring out and hoping your peripherals catch something. Guards typically have discrete areas they cover, and within those boundaries you can check for a certain number of people and boundary crossings. By doing things like mentally running through the list/number of people in your area and noting the higher-risk ones, it's a little less mentally taxing than you might think. Combined with some overlap between guard stations and regular rotations between areas, coverage can be pretty good. The real difficulty comes in when you start adding numbers of people- the boundaries become less clear, the number of high-risk swimmers increase, and the total cycle time through everyone you're covering increases such that your margin of error between a situation presenting and your time to react starts decreasing. Anecdotally, I always found the behavior changes from a normal to tired swimmer to be one of the easier parts. Lack of forward motion, intense focus on the activity of swimming itself, falling behind from a group. Again, it's more difficult the more crowded an area becomes.