Let's say that after decades of development there are now cities on Mars. Most of their facilities are located underground in order to protect them from radiation and small meteorites, as well as providing additional insulation from the Martian climate. However, buildings that require access to sunlight, such as agricultural facilities, would be left vulnerable.
So, are there any issues with using water reservoirs (with a clear top and bottom) as radiation shielding to protect the upper layers of the colony? And would there be problems with using that water for irrigating crops or drinking water?
Also, I should mention that the water is kept circulated and heated to stop it from freezing. Preventing it from bursting the pipes or the reservoir itself.