Like others who commented I worked as a life guard and water safety instructor when I was younger. (Everything I am saying pertains to the US only.)

I rescued quite a few children, it's hard to see in this video because the video quality is poor and the camera is at an angle that is worse than what the lifeguard in the video is seeing. Overall this victim is fairly active and should not have been very hard to spot in person for a well trained lifeguard. (And they did spot the child.)

This is a weird video because:

There are lifeguard(s) but yet the pool is full of non coast-guard approved tubes and floatation devices such. Most places with well trained lifeguards would not allow this. They don't work, can be more dangerous than no PFD, and they make it harder for the lifeguards to see. The worst drowning incident I witnessed involved a child in a tube who got flipped upside down and couldn't get out of the tube or flip back over. (I was not a lifeguard yet when I saw that.)

I think things from my perspective are in a terrible state in terms of water safety compared to 20 years ago.

Something must have changed with insurance liability, as most places just don't even have lifeguards. Resort pools I see these days are designed in a way where sight lines are so poor lifeguards/parents cannot even see children in the pools unless they are in the pool and stay within 10ft of the child. Very different than things used to be. Pool designers have reduced depth & eliminated diving boards resulting in a false sense of security. Meanwhile the pools are no longer even sufficient to be used for teaching up to a point where a person can be considered a swimmer. I just got back from vacation and the resort we stayed at had a pool which absolutely terrified me. I was 100% in lifeguard mode the entire time my child was in the pool, and the pool was so bad I couldn't sit in one spot and see him, I had to walk the edge of the pool to maintain sight lines. (The pool in the video is not like this FWIW)

Fewer young people are supposedly physically fit and able to get to advanced swimming levels and pass tough standards like Red Cross. There are fewer places that even have Red Cross accredited programs these days as a result. Red Cross level instructors command high pay, and most places teaching swimming lessons these days are money making businesses that pay instructors near minimum wage and try to make the franchise owner wealthy. This is a relatively large change from non-profit Red Cross programs back in the day.

Red Cross has always refused to act as insurance for pools/resorts/water parks, and alternate private organizations now certify lower quality lifeguards & swim instructors and we have new things like "Shallow Water lifeguards" that can be paid minimum wage. These alternate private certification orgs train to a lower level but do act as insurance so they're very attractive.

I have a 7 year old, he's been through 4 private orgs so far. None have had Red Cross accredited programs. All of them have been super expensive but they're the only choice available. 3 of them did not have deep enough water and their instructors were not trained at a level for teaching to a full "Swimmer" level. None of the programs seem to focus on water safety and have strange practices like trying to teaching 5 year olds the butterfly and other high energy/low safety strokes without teaching elementary backstroke, breaststroke, sidestroke, etc.. which are more useful in emergency water situations. Most of the instructors I've seen teaching my child show poor enough form they'd have not passed a Water Safety course 20 years ago.

The whole thing is a giant mess. I have been considering getting re-certified to take over finishing my child's swimming lessons, but the course is hard to take these days. Which also explains why not enough 16-20 year olds manage to take it. And there are almost no pools left to use that are not privately owned and have deep enough water.

Also at least by the old standards when someone who was a Red Cross WSI calls someone a swimmer we're talking about a pretty high level. Someone who can't swim for 30 minutes to safety in deep water is not necessarily a swimmer IMO. Maybe standards have reduced. But that was a requirement at one point. And this is not something that requires elite physical fitness or stamina when you are trained to swim well. Some of the strokes are barely more physically taxing than walking if you're proficient. Non swimmers get a false idea about this because they mostly see competitive swimming which uses the taxing/fast strokes.