In the standard game, Minecraft's water behaves like everything else in the game: like a block. DJoslin's "FiniteLiquid" mod, however, turns it into actual water, which can have devastating results.
The mod changes the behaviour of water in the game by giving oceans an "infinite" amount of the blue stuff. What this means is that if you break a "wall" or piece of terrain that's holding back the ocean, the game's water will flood in realistically.
And I'm not talking "flood a mine" kind of flooding. I'm talking "bury half the map in water" kind of flooding, which you'll see in the above vid if you head to around the 3:55 mark.
Accompanying the mod are some pipes and pumps so you can mess around with the water some more, give your manor house that cascading water feature you've always wanted.