Wetstone is a stone brick block that acts as a source of water. This means that you can build a solid structure with water built right into the walls but it functions wonderfully as an alternate source when farming.
It also means you can carry a significant amount more water on you when you can stack 64 blocks compared to 1 full bucket.
The block will provide the same amount of water for a fully functional farm without the risk of spillage or flooding.
And you can even craft machines capable of converting lava to cobblestone or obsidian since the block can be moved via piston as well!