Drowning is very fast, seconds not minutes like in the movies. People in distress can take minutes before they are actively drowning. Active drowning is silent, they will not be yelling for help. It looks like the person is "climbing" or pushing down at the water. They will be vertical in the water and may be "bobbing", going underwater and resurfacing. They will have their head tilted back parallel to the surface of the water.

If you see someone go under in open water keep looking at where they went under while calling for help, don't take your eyes off it. If you are the only one who saw them go under, your job is to direct others to where they went down. In open water it's very hard to find people because the bottom isn't visible.