When was the flood?

In the calendar developed by Archbishop James Ussher (who calculated that the world was created in 4004 BC), Noah's flood happened in 2348 BCE.

By comparison, there are Bristlecone Pine trees alive today that are dated to be 500 years older than this.

Of course there are many ancient stories of 'the flood' from different cultures.  Some are almost certainly older than the Old Testament version. Which one should we believe in if we don't accept that the bible is the literal word of god?

But more importantly where did all the water come from and where is it now?  And how did the koalas and kangaroos get back from Australia to get on the ark?  Or maybe they weren't there in Australia originally, but why did the amazing rafts of floating vegetation take the koalas and kangaroos only to Australia after the flood and not leave their kind anywhere else along the way there.

How can anyone really believe in this as a rational literal story? The fact that some people do believe it only strengthens my disbelief.