The situation on which my question is based, two mermaids are imprisoned in an underwater prison unit. This city is on top of a plateau. Hence, mers mostly move horizontally like humans and often vertically, depending on the landscape. They can also harness the power of the ocean to perform elementary magic.
Now, one of the mermaid is imprisoned as she couldn't provide a valid identification after immigrating to this place. For reasons relevant to the plot, she is in excruciating pain and paralyzed from the waist down, covered in blood. She cannot swim at all. Someone has to carry her to escape. The other mermaid is physically healthy and is imprisoned for not paying her taxes. She has been in jail a couple times before and is quite familiar with the place. In what unit of the prison, is it possible for these two types of criminals to be imprisoned together?
Keeping in mind the water pressure, current flow, types of materials that can be used, what kind of prison should be built underwater for merpeople who have elementary magic powers (mostly water magic) and can move in 3 dimensions? How can a half paralyzed and an able bodied mermaid escape this kind of prison built to trap their kind?