This page offers some information on the DIY recipes for watering cans.
Flimsy Watering Can DIY Recipe
To obtain the recipe, you’ll need to have donated 4 specimens to Nook in Resident Services. He will then give you the recipe as a reward.
To craft the watering can, you’ll need 5 pieces of softwood. This watering can is not very durable and will break after 20 uses.
To unlock the recipe for the standard watering can, you’ll need to buy the Pretty Good Tools Recipes from the Nookstop, which costs 3,000 Nook Miles. To craft this watering can, you need to craft a flimsy can first, then use that alongside an iron nugget to craft a more durable watering can.