From cub scouts when I was a kid:
1 inch keep off
2 inches 1 may
3 inches small groups
4 inches ok
Sounds reasonable. I fell into a creek while ice skating as a kid. Not such a big deal because it wasn’t too deep and I didn’t get swept under the ice - but under the wrong circumstances you can easily die. It was a really cold day, and everything wet was frozen within a minute. When I took my pants off they stood up by themselves.