By the way, "Block.blocksList[l].isBlockReplaceable(world, i, j, k)" won't just explode on water.
It'll explode on a whole host of other things too. Like snow, lava, tall grass, and many mod blocks such as mod added liquids and gasses. I even have a mod that creates a solid block you can walk on, but has no outline, no texture, and can be replaced by other blocks.
Dimensional Doors uses a isReplacable block for the volume-material of its pocket dimensions.