From the RuneScape Wiki, the wiki for all things RuneScape
A Watering can is an item that members use in the Farming skill to water vegetables and flowers as well as potted tree seeds, but not other farming patches.
Watering a patch will stop it going bad for that growing cycle. If players have fully-grown flowers protecting the surrounding vegetables, they do not need to water those vegetables. This is a better alternative than having to spend additional money and use up more inventory space to carry plant cure for vegetables that fall sick.
It can hold up to 8 doses of water. After players use them up, they will need to refill it at any water source. Players cannot directly refill the watering can from a well, but can fill a bucket from the well and then use the bucket of water on the watering can to fill it. Players can also transfer water from one watering can to another by using one on the other. If a watering can has 3 doses and added to one that has 4, players will end up with an empty watering can and a watering can with 7 doses.
Players can buy a watering can from farming shops. Full watering cans are also rares drops by a farmer. Watering cans cannot be withdrawn from banks in notes, as they are not tradeable.
||This watering can is completely full.
||This watering can is almost completely full.
||This watering can is three quarters full.
||This watering can is just over half-full.
||Some would say this watering can is half-full, others half-empty.
||This watering can is just under half-full.
||This watering can is three-quarters empty.
||This watering can is almost empty.
||This watering can is empty.