Extra Water Cans can be bought for 625 
Brass from Hawthorn after finishing the Water Can Thief (HT0002) quest. It is essentially the same supply item as Empty Water Can and both of them stack in the inventory.
The option to buy an extra water can only appears if you have enough Brass and are not currently growing anything in any of the planter boxes.
Buying Extra Water Cans[]
Multiple Extra Water Cans can be bought, each one for the same amount of Brass. The player can fill all of them at the same time using the Water Pump. Having more than one Full Water Can filled with water, it is possible to water more than one plant during Gardening.
Efficiency[]
Considering the price per performance ratio for watering all 5 plants:
- 1 Water Can - 5 usages of Water Pump - free (quest reward)
- 2 Water Cans - 3 usages of Water Pump - 625
Brass - 3 Water Cans - 2 usages of Water Pump - 1250 Brass
- 4 Water Cans - 2 usages of Water Pump - 1875 Brass
- 5 Water Cans - 1 usage of Water Pump - 2500 Brass
The most efficient option is to have 3 Water Cans, as the total cost is only 1250 Brass and it allows to water all the plants in the garden with only two usages of the Water Pump.