This is a story related achievement which will unlock with natural story progression.
Once you enter Goulston Street Homes will tell Watson to gather water. You will then take control of Watson and have to find several items before you can collect the water from the barrel behind you.
Run down the street until you see several stands, you will need to collect:
Once you have these items combine them in your inventory. You will find a watering can up on a ledge above, move the box next to the stable and climb up. Use the hook you just attached to the rod to collect the watering can.