In fact very easy to do.
You need, of course, the "Financial districts" DLC for this to unlock.
Banks unlock at Level 2 of the Stock Exchange main building, so you will need to earn 50,000 Cim-$ for it to upgrade and unlock banks.
Once you've reached level 2 of the Stock Exchange building, select the small bank from the "Banks" tab and place it near water, either a river or a shoreline.
If you own the "Natural Disasters" DLC, simply rain down a meteor of small size (let's say size 2 or 3) into the water to create a wave, which will eventually hit the shoreline and flood the bank building, unlocking the achievement.
If you don't own the "Natural Disasters" DLC, you'll need to have some water rise by different means.
You can either retain and release some water by building a dam (either from terraforming or a dam power plant) and then level it to unleash the water. Or you mess with the shoreline by building quays or moles and deleting them afterward. It can also cause flooding.
There are a lot of ways to achieve this. Easiest is by using a natural disaster.