I'm sure you've heard meteorologists say "turn around, don't drown." We couldn't be more serious about this topic! Each year, people get swept away by moving water that they think they can drive or walk through. We frequently remind people that 6+ inches of water can stall your car, and easily knock a person off their feet. A foot of water will likely float your vehicle, and once we're talking 2 feet of water, well your car is now a boat likely to sink.
I think what's challenging for people during flooding events is that even knowing the facts that I just listed above, many people think they can eyeball the depth of the water. The truth is during a flood, the depth of moving water is changing (especially a flash flood.) The moving water has also picked up a ton of sediment debris, so it's very difficult to see through the water to get an idea of depth.
Third, the velocity of the moving water will change with differences in topographic slope, and the velocity will also change with changes in volume to the water that contributing to the flooding. As we all know, the faster the water moves, the easier it is to bulldoze through objects, and sweep cars and people away. Are you able you eyeball the speed of river? I know I'm not able to just know the general velocity of moving water without measuring it. So, that unknown regarding velocity of moving water, is another reason why it's too dangerous to take a risk and assume you can walk or drive through flooded areas.
One other thing that's important to mention is that the fastest flowing areas within a flooded river, for example, are not at the margins of the river. The fastest-flowing areas are also not at the surface, but instead toward the middle. You can't see the middle interior portion as we stand to the side of a flooded area. So, by the time someone reaches this midpoint, and finds out that the water is moving too fast to control their vehicle, it's typically too late and too difficult to get back to the shore.