开水 is the general term of boiled water, it's usually fresh boiled. 开 (boiled, not opened) indicates the temperature of the water and it doesn't mean water cooking.
热水 means hot or warm water, it must not been boiled before.
烧水 means water cooking. 烧 is the verb and 水 is the object.
There are many food cooking terms associated to water or steam. For example steaming 蒸， you can see the 4 dots there, which symbolize the fire。包子 is one of the most steamed Chinese food, it's steamed stuffed bun.
Another example is poaching 煮， which contains very similar structure. One famous food is called 水煮鱼 poached sliced fish in hot chili oil. Have a try ;-)