I watched a few videos and found myself analyzing how good swimmers each were and got a lot better at spotting danger by noting who looked good and checking on them less often. So I don't think you need to KNOW the kids over multiple visits, but just constantly scanning for and assessing skill level.
I agree, there are some that appear more competent and others that seem less so. Granted, all are capable of panicking and needing assistance, but prioritizing does look like an effective strategy.
