A conversation in class today

Me: Let us go jogging in the evening today
Sunil: yes, we must
Me: We will definitely go
Sunil: Surely go
Me: Come hell or high water!
Sunil: (worried tone) Hmm, come hell or high water?
Me: (after a pause) Okay, if hell or high water come, we wont go, otherwise we will definitely go.
(after a longer pause)
Me: What the hell is high water anyway?

Any idea?