Today I went over at my friend's house. Two days ago, we had a huge rainstorm here, lasting all day long. My friend's house is small, so he stocks most of his stuff in a cellar. Yesterday I met him, and he told me that about the cellar had been flooded with about 20 cm of water, enough to completely flood a lot of comic books, and his whole AD&D collection.

Now, he is not a big collector, he mostly collects at AD&D 2nd Edition, with a bunch of basic manuals, Dark Sun and Ravenloft boxes accessories (he only has a couple of modules), some Forgotten Realms, and a few extra stuff. Not caring too much about condition (for example, as a present I gave him a Kara-Tur boxed set with the box in crappy status, but he was happy nonetheless). Probably, put together, no more than 1500 dollars worth. However, seeing his house with manuals and maps drying everywhere, with him taking the manuals and separating pasted pages one by one… that was almost a heart attack for me. I can really understand why yesterday he was going around pubs and getting drunk… Probably I would have done the same.

I've been flooded before, and it is shocking how thorough damages from flooding can be.  About the only things which are salvageable are glass, plastic and ceramic items.  You can save furniture if you get to it quick and has no presswood, but flooding is horrific.  At least with a fire, you aren't tortured into looking at all the damaged stuff and hoping it can be saved only to realize it can't.  Bummer.  

If you haven't lived through a flood, it's hard to imagine how devastating it can be.  Truly awful.  Sympathy to your friend.


Regarding containers, you might try two-drawer filing cabinets.  
They are squared-off and you can find types where the drawers seal up very well.

My two cents'