"Come hell or high water" makes clear that some future event will happen and no other event will stop it from happening. Example: "Will you be at the family reunion next year?" Answer: "Yes- we'll be there, come hell or high water!" Even if events became totally chaotic ("hell") or there were natural disasters ("high waters") the future event would still occur. 
*The word "hell" is informal and should not be used in polite situations.