how to make bag filled with water that can be moved by physics?

So I am making a 2D game and in one level I would like to have a fish traped in bag of water and when the fish hits the corner of the bag it should roll with physics, but I dont know how to implement it.

The only thing i can think of is making the bag out of many colliders and make the wather out of really small objects that would colide with the bag but I am pretty sure its not the right way to do it...

Any help appreciated.