沸水指正在沸腾的水（boiling water） 开水指已经烧开了的水，不一定正在沸腾，也可能是凉的（boiled water）

沸水是正在沸腾的水 很热 开水是沸腾过的水 。。可能是热的 也可能是变凉了的

沸水 is rare in spoken Chinese 开水 is very common, could be boiling water or boiled water,you need figure it out by context. eg,1: my feet had been burnt in 开水 few years ago/ boiling water. 2:"hi,waiter,can i have some 开水？” --Now,the water couldn`t be boiling whatever how hot it is. and it could be hot 开水，could be cold 开水。the resturant will simply bring you a pot and some glasses... just suit yourself

两者没有本质的区别。沸水用在书面语，开水用在口头语。

没有区别。 沸水属于专业术语，如化学、医药行业等； 开水用于日常生活中。