Interestingly, undercurrents aren't as common as people think.
Most currents can't pull you under the water. There are notable exceptions such as obstructions, dams and rapids which can form "keepers", or sudden depth or width changes that can make the water flow extremely fast and turbulent beneath the surface (such as the strid), but those sorts of situations aren't that common.
Most drownings in rivers and lakes where someone was apparently pulled under are actually due to cold water shock. The water beneath the surface of a river can be extremely cold, which can immediately paralyse even the most experienced swimmer.
Worse, sometimes the water can be a lot colder than other times, or colder in the middle than at the sides, meaning the river could be safe sometimes and not other times, and it might seem fine when you first get in, only to find you rapidly fatigue as you move further away from the bank.
That's why I'm posting this advice, they are not common if you count all lakes and all rivers in the world but they are very common in the rivers and lakes near where I live for example.
That's why most drownings around here happen to immigrants: local people know that it is crazy to go swimming in some places, because they know that something bad happened to a lot of swimmers over the years in those points, but if you don't know that history it is easy to make that possibly fatal mistake.
Most currents can't pull you under the water. There are notable exceptions such as obstructions, dams and rapids which can form "keepers", or sudden depth or width changes that can make the water flow extremely fast and turbulent beneath the surface (such as the strid), but those sorts of situations aren't that common.
Most drownings in rivers and lakes where someone was apparently pulled under are actually due to cold water shock. The water beneath the surface of a river can be extremely cold, which can immediately paralyse even the most experienced swimmer.
Worse, sometimes the water can be a lot colder than other times, or colder in the middle than at the sides, meaning the river could be safe sometimes and not other times, and it might seem fine when you first get in, only to find you rapidly fatigue as you move further away from the bank.