The case about a global flood is this: Jesus believed in it and spoke of it. If I reject the flood, then I reject Jesus.
Well, ignoring the fact that this is a horrible reason to believe the flood. It's not necessarily true, even if you believe that what the bible quotes Jesus as saying is somehow reliable.
You are, probably, referring to Matthew 24:36-40
36 “But about that day or hour no one knows, not even the angels in heaven, nor the Son,[a] but only the Father. 37 As it was in the days of Noah, so it will be at the coming of the Son of Man. 38 For in the days before the flood, people were eating and drinking, marrying and giving in marriage, up to the day Noah entered the ark; 39 and they knew nothing about what would happen until the flood came and took them all away. That is how it will be at the coming of the Son of Man. 40 Two men will be in the field; one will be taken and the other left.
First of all, I make references to movies pretty often. For example I might say to my family: "I set my phone up so I can track it online, just like Sherlock did with that murder victims phone."
I can safely use that example with my family because I know they saw that episode. And I don't have to point out that I don't believe it actually happened.
Jesus could easily have used the Noah story as an example because he knew his audience would be familiar with it. That doesn't have to mean that he believed it actually happened. He also doesn't mention anything about it being global.