Seaweed is a block found in an ocean biome.
Seaweed can be crafted into fiber by placing it in the crafting table one at a time.
Seaweed grows when it is planted into a water source block on sand.
It can grow only if there is a water block above it (any type), which is then replaced by a seaweed block. This causes a water update.
Seaweed blocks are not water types, and do not leave water when harvested. To maintain a seaweed farm, a secondary water source must be kept free of seaweed to supply the water into which it can grow.