When you say ‘Come Hell or High Water’ you mean that you are willing to do whatever it takes to overcome difficulty or obstacles.
Example of use: “I’m going to make it to the dance on Saturday, come hell or high water!”
The expression ‘come hell or high water’ originated in America. The first printed reference comes from an Iowa newspaper, The Burlington Weekly Hawk Eye, dated May 1882: “Since that time the best of my friends had become enemies and strangers have become friends. The devil had broke loose in many parts of the country and keeping up with the old saying, we’ve had unrevised hell and high water, and a mighty heap of high water, I tell you.”