Ok, let's break it down.
"
2 Peter 2:5, And spared not the old world, but saved Noah the eighth
person, a preacher of righteousness, bringing in the flood upon the world of the ungodly;"
God didn't spare the world in unbelief, but He saved Noah.
Noah was a believer, a preacher of righteousness that they mocked and would not heed to his preaching.
So God saved Noah but the world of unbelievers were taken away with the flood.
What did the water do to save Noah? Absolutely nothing as far as the saving of the soul, Noah was justified by faith, it was a physical saving from drowning. And there is the symbol, "in the example/symbol of the water saving Noah from the flood physically, Peter is showing us how water baptism is the symbol of our true spiritual baptism saving us in the Spiritual realm.
Remember, Peter is using all of this as an example of baptism in his day and ours.