If you haven't already explored the literary genre of Afrofuturism or Caribbean futurism, it would be good to read some exisiting literature. The Deep by Rivers Solomon depicts merfolk as the descendents of pregnant African women thrown overboard from slave ships. Tentacle by Rita Indiana doesn't explicitly contain merfolk, but it does focus on water deities and mystical properties of the sea (as well as time travel and colonialism) in order to represent indigenous perspectives.
Sure, control an oppressive and evil gang with an oppressive and evil government. That should work out well. Populist leaders with the power to lock up anyone seldom abuse their powers /s