The tub is a container that looks fairly average, but in truth it's one of the more unique items in the game. The empty tub weighs 10 stones.
They can neither be bought nor crafted. To obtain one you'll to collect an empty Glass Pitcher, then head to the stables in any major town or city (such as those just west of Britain). Use the pitcher on a tub filled with water and the empty item can then be picked up. New water tubs appear each day.
Use a Pitcher of Water on a tub to pour the contents into it. Doing this will cause the tub to fall to the ground again, and make it impossible to move until it is once again emptied - In fact, the tub will not disappear no matter how long it is left there, unless someone comes and removes the water from it.
Each tub can hold the equivalent of 10 filled pitchers. If this is the amount poured in, then this is the amount that must be removed in order to move it again.
Many players have been known to "decorate" certain areas will tubs, or perhaps use them to spell out their names in remote areas of the game world. Placing a large amount of tubs in a well populated area typically results in their swift removal by those concerned about game lag.
While not commonly seen, many players falsely assume that empty tubs are rares, even to the extent that some unscrupulous players offer them for sale as such.