A reliable source of water for farming and way to raise fish.
When craft, the pond is empty, and will be refill when rain come. You can fill watering can on fresh water pond. You can turn fresh water pond into salt water pond by drop salt crystal into it.
You can raise fresh water fish and eel in fresh water pond, and ocean fishes in salt water pond.
You can put bull kelp in salt water pond, and turn them to reliable source of kelp in cave.
A pond will reproduce fish when there atleast 2 fish on the same kind on the pond, and maximum of 10 fish per pond. Maybe luxury fish pond can hold to 20 fishes.
feed fish food to fish pond will speed up progress.