"Come Hell or high water" vs "Lord willing and the creek don't rise"

They don't really mean the same thing.

Come hell or high water

That means it will get done no matter what.

God willing [and the crick don't rise]

Whereas that means it will get done only if something doesn't prevent it.

If hell comes then you've got to oppose it to get into heaven. But on the contrary, if something is God's will then you have to let it happen.

So really the first is saying "not even the devil can stop me" whereas the second is saying "God will decide if this happens".

Similarly with the reference to rivers. The first defies the river, the second acquiesces to it.

These phrases, therefore, are opposite in meaning.

Come hell or high water indicates that something will be done regardless of difficult circumstances or problems.

Lord willing and the creek don't rise indicates that a positive outcome depends on God's intervention or blessing.

Creek was in reference to the Creek Indian tribe rising up and going to war, not a body of water.

Come Hell or high water. Yes it seems on the surface to reference overcoming difficult circumstances. But below the surface the saying references overcoming the ultimate circumstances. The phrase can be broken into three parts. Part one: Hell. Hell references the devil's dominion and the ultimate enemy of mankind. The second part: or. Or leads into a contrasting circumstance that is implied as worse than part one may be, but also hints at a relationship between part one and part three. So this leads us to part three, but what could fulfill the requirements of being related to the idea of hell and also a worse situation than being in hell? Part Three: High Water. High Water references the biblical flood GOD sent down to destroy the world. As the wrath of GOD it is both related to the idea of hell as a very bad situation to be in and also that it is far worse than going to hell.

You are confusing the determination with positivity.

The first statement, is in fact negative, it expects difficulty, perhaps what would normally be considered insurmountable difficulties. It expresses determination to overcome any such difficulty, but that very expression of determination is an oblique warning of the possibility of failure in face of the expected problems.

The second statement, is positive, it doesn’t expect any difficulty, and if difficulties occur, expect’s them to be easily dealt with. Only truly unforeseeable and insurmountable problems could prevent a successful conclusion. Such as God personally deciding that it won’t happen.

You would not say “I will get a gallon of milk from the store, come hell or high water” except as a joke, there’s no reasonable difficulty where that would be appropriate. You could say “ I will get a gallon of milk from the store, god willing and the creek don’t rise” without joking.

You would not say to someone fighting cancer, “You can beat this, God willing and the creek don’t rise”, it’s too positive about the outcome (not to mention that it may feel like God already has it in for them). It brushes the expected difficulty aside as if the task ahead was no more difficult than getting a gallon of milk from the store. You absolutely could tell someone fighting cancer, “I know you can beat this, come hell or high water”. You expect a good approximation of hell to happen, but know that they will give everything they got to get through it.