If I remember right, 7 cm of clear ice is regarded as the minimum for safely walking on ice. 4 cm can work out, but then one has to know very well what one is doing and carrying security equipment.

I did buy some equipment a couple of years ago as I like skating, and also want to take the kids with me for skating. However, it's rarely possible to go skating here (Oslo, Norway) as the lakes usually are covered with snow. Previous winter I managed to take the kids for skating on the local lake a couple of times. People on the shore was yelling at us as they considered the ice to be unsafe, but I did my research, it was safe enough.

The sea ice is usually less safe, there are more waves on the sea, salty water doesn't make the same kind of ice as fresh water, salty water requires less temperature for freezing. Sea currents also plays a role - in the north, were I was grown up, we never had ice on the sea, but in the Oslo area we have some, especially in shallow bays. There is a basin here protected by quite some islands, almost every year it freezes up making it safe to walk between the islands. They even drive heavy machinery there to make skiing tracks. Some years ago, on a sunny and warm day in March I was out with the boat and was headed for this basin, completely unaware it was still frozen. I was surprised when I came to the ice, tried to break my way a little bit in the "right" direction, but decided to give up when when I saw someone skating on the ice ...

The most extreme I did was probably in the youth, in 1997 in the mountains around Tromsø we crossed a lake by skis in august.

We also have an online database of ice observations as well as accidents.