Why is hot water called 开水? Why is hot water sometimes called 开水? What is the meaning of "open" in this context? How is this different than saying 热水 or 烧水?

Is it something to do with how the water was made hot, kind of like how there are sometimes different words for frying something (炸，炒，etc.)?
One of the meanings of 開 is 沸騰 (boiling) in 《漢語大辭典》開32.
It's nothing to do with "open" or "how".

開水 boiled water
熱水 hot water
燒水 to boil water


When the water is still very hot, we can use 開水 or 熱開水 as hot water.
When it is warm, we use 溫水 or 溫開水.
When it's at room temperature, we use 冷水 or 冷開水.
When it's below, say, 10℃, we use 冰水 or 冰開水.


開 can indicate that the water has been boiled. Without 開, it may or may not have been boiled.

The above Chinese words are used in Taiwan. They are probably different in the mainland.