Unlike the many shovel upgrades that are unlocked by working and performing different tasks in your garden, watering cans are upgraded through purchases.
Costolot's Store provides the Tin and Gold watering cans, while the remaining three upgrades - the Glass, Everpour 5000 and One Pour Wonder watering cans are available from Ivor's Store.
Thankfully, you only need to buy the One Pour Wonder Watering Can (4125 chocolate coins) to unlock the achievement. Upon purchase of the One Pour Wonder, all the lesser watering cans become "sold out", so if you want the information for each watering can in your journal's database, be sure to purchase the One Pour Wonder last.