The Shaldon prison

A prison which polymorphs its prisoners into forms more suitable for selling into slavery.

The fishermen of Shaldon always enjoy a plentiful catch thanks to the town's good relationship with nearby merman settlements. The aquatic creatures guide their fishing boats to shoals where they can spread their nets. In return, the town maintains the prison and uses it to jail criminals and other unwanted visitors.

The cells are short of amenities - there's no point putting bedding down when it'll just wash away. The cliff-side prison is built so that the lowest level, where the cells are, is below the high tide line. 

The cells have spells built into their very bricks. First Sleep, to keep prisoners from panicking as the waves start to splash into their enclosures. Then Polymorph, to change them into mermen. When the tide reaches its peak, The Shaldon mermen swim in to collect their new slaves. A life of cultivating kelp awaits them.

Prisoners who make their saves and resist the Sleep spell aren't as lucky. If they're not asleep, Polymorph doesn't trigger. They're likely to drown when the air pockets at ceiling height turn stale.

There are three cells the guards don't use. They may never have been used. Instead of polymorphing the occupants into mermen, they change them into sharks. Each door is closed from the outside with a handle that must be twisted upward to open them. Each handle has a wooden float secured to it by a cord, so the doors unlock by themselves when under water. The guards don't talk about these cells.