Water Container is an interactive part in Scrap Mechanic.
Connect it to a Vacuum Pump that is under water to fill the Water Container with water
The main purpose of the Water Container is to provide Water Cannons with Water, since its inventory hold a very limited amount of Water.
To make an interactive part draw Water from the Water Container, connect them with the Connect Tool.
You can use a Water Bucket on the container with water in it to refill the bucket.