Why is hot water called 开水?
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.