why did the secret annex inhabitants dicuss what to do incase of a flood

The inhabitants always needed to know what to do if some emergency came along. They did not want to get found by the Nazis, so they esentially needed a plan of escape for every possibility. Since the house was by the canal, flooding was just another eventuality they needed to plan for.

The papers notes that if the British landed in Holland, the Germans planned to flood the city if they had no other way to hold and protect it. The discussion initiated because of this ibnformation but was basically a round robin of friendly banter.

This tricky question elicited a variety of responses:

"It'll be impossible to walk or ride a bike, so we'll have to wade through the water."

"Don't be silly. We'll have to try and swim. We'll all put on our bathing suits and caps and swim underwater as much as we can, so nobody can see we're Jews."

"Oh, baloney! I can just imagine the ladies swimming with the rats biting their legs!" (That was a man, of course; we'll see who screams loudest!)

"We won't even be able to leave the house. The warehouse is so unstable it'll collapse if there's a flood."

"Listen, everyone, all joking aside, we really ought to try and get a boat."

"Why bother? I have a better idea. We can each take a packing crate from the attic and row with a wooden spoon."

"I'm going to walk on stilts. I used to be a whiz at it when I was young."

"Jan Gies won't need to. He'll let his wife ride piggyback, and then Miep will be on stilts."

So now you have a rough idea of what's going on, don't you, Kit? This lighthearted banter is all very amusing, but reality will prove otherwise.