Rare deco/container Tub

Super cool item! How do you fill it and empty it with water though?

With a pitcher from an innkeeper. You can even put alcohol in it and it comes back out as water... Kind of a reverse Jesus. It actually stores an amount of water, as well, it's not just binary. For instance, if you fill it with two pitchers, you can take two pitchers worth out of it.

And what happens with the items inside? Is it still a usable container with water? Can the items still be moved?

If there are items in it, water will not go in. If there is water in it, items will not go in.