So every human had turned pretty bad (save Noah and his family), so God decided he needed to destroy all the living beings and start over basically. He commanded Noah to build a ginormous ship to carry two of every creature.
Prove it. Where is the evidence of this "Noah?" Where is the evidence of this unimaginably huge boat? Archaeological, paleontological, historical, anything...nope, not a shred. Why is this "Noah" character extremely similar to multiple other mythical characters of other ancient civilizations (Gilgamesh particularly)? Why is the story of Noah's Ark extremely similar to multiple other global flood myths of other ancient civilizations (again, Gilgamesh in particular).
This "Noah" character is said to have lived to be 950 years old -- very realistic. Even if you claim that it's a result of mistranslation, he's still listed at 83 years old -- also unrealistic given the time period.
There is literally zero legitimate historical evidence of such a flood occurring in the time period. There is literally zero legitimate paleontological evidence of such a flood occurring in the time period. There is literally zero legitimate archaeological evidence of such a flood occurring in the time period. There is literally zero legitimate historical evidence of such a physical being as Noah existing in the time period.
The Bible is not a legitimate historical source when nothing can be cross referenced to confirm what it says on this issue.
I wasn't exactly there, so I can't tell you everything.
You can't tell me anything. You have nothing to support anything you say in this post except a book from people who would be amazed by a wheel barrow.
Whether Dinosaurs got left behind, whether they got on and then became extinct after, or whether they were there and then devolved is up for debate, but whatever.
It's not up for debate at all among sensible people. It is only up for debate among silly, naive young earth creationists.
So a mating pair of each animal was put on board (by God leading them there), and then it rained for 40 days and 40 nights until the whole world was covered (even Mt. Everest). All the humans on earth died, and all the leftover animals as well. Noah and his family rode out the storm and made it safely to land. All the animals mated, and a ton of years afterward, we have all the species today.
Again, show me even a slight amount of historical, archaeological, or paleontological evidence and then we can discuss the subject a bit better.
Why is it so hard to believe? Because it goes directly against the history we know. It goes against the age of the Earth. It goes against logic. It goes against evidence.
I don't see why people think that would be impossible when they would easily accept the theory that from a whole bunch of random chemicals, a single cell was formed (by CHANCE), and reproduced (by CHANCE), turned into a different thingy mabob (by CHANCE), turned into bigger thingy mabobs (by CHANCE), came out of the water and survived and became a different animal (by CHANCE), mutated, mutated, mutated and mutated some more (by CHANCE) until it became every species of today, all living simultaneously (by CHANCE). Wow, lot of lucky happenings there.
If you're going to use the term "thingy mabob" when describing evolution, it's pretty likely you don't have the slightest clue what it really is or what the theory encompasses.
and also, how are there sea creature fossils on mountains if the sea wasn't up to the mountains?
You assume the water rose to that level, rather than those mountains actually being at a lower sea level in the past. One proposal is supported by scientific evidence of the Earth and every sensible geological historian, the other isn't.