You can watch someone drown right in front of you if you don't know what to look for.
The first paragraphs of the article "Drowning doesn't look like drowning" are helpful / horrifying (same name as item linked above, but different website) -
"The new captain jumped from the deck, fully dressed, and sprinted through the water. A former lifeguard, he kept his eyes on his victim as he headed straight for the couple swimming between their anchored sportfisher and the beach. “I think he thinks you’re drowning,” the husband said to his wife. They had been splashing each other and she had screamed but now they were just standing, neck-deep on the sand bar. “We’re fine; what is he doing?” she asked, a little annoyed. “We’re fine!” the husband yelled, waving him off, but his captain kept swimming hard. ”Move!” he barked as he sprinted between the stunned owners. Directly behind them, not 10 feet away, their 9-year-old daughter was drowning. Safely above the surface in the arms of the captain, she burst into tears, “Daddy!”
How did this captain know—from 50 feet away—what the father couldn’t recognize from just 10? Drowning is not the violent, splashing call for help that most people expect. The captain was trained to recognize drowning by experts and years of experience. The father, on the other hand, had learned what drowning looks like by watching television. If you spend time on or near the water (hint: that’s all of us) then you should make sure that you and your crew know what to look for whenever people enter the water. Until she cried a tearful, “Daddy,” she hadn’t made a sound. As a former Coast Guard rescue swimmer, I wasn’t surprised at all by this story. Drowning is almost always a deceptively quiet event. The waving, splashing, and yelling that dramatic conditioning (television) prepares us to look for is rarely seen in real life."
The article goes on to clarify the difference between instinctive drowning response and aquatic distress and that if someone can't respond to an "Are you OK?" get them out of the water.
Jo