The water brush type is used to fill in around other objects. Water can exist as both a regular block and as a flag that indicates water exists in the block. The Water Flag is used to create water that fills in around other blocks. For example, the water flag is used to create water around a "Reeds" block in the Farm level or to fill in water around beveled walls. When painting using the water brush type, you will only affect the water flag and nothing else.
Left-click to paint water, right-click to remove it.
The depth of the water in a flagged block is dynamic. If there is water above the block, it will fill the whole block. If there is no water above the block, it will only be 75% filled.
The only other brush type that affects the water flag is the Copy Brush Type.