Man, shark mafia is such a weirdly specific and cool little corner of books. You've got the classics like 'Mako' by Jaws fanfic writers trying something new, where a loan shark metaphor gets taken way too literally. The whole thing becomes this underwater 'Godfather' situation with territorial disputes over fishing lanes and offshore rigs, which is ridiculous but also kind of brilliant.
Honestly, the thrills are less about car chases and more about deep-sea chases in submarines, or tense negotiations on a yacht where someone gets tossed overboard for the 'sharks' to handle. The ocean crime angle lets authors play with smuggling routes, sunken treasure, and biopiracy, which you don't get in your standard city-based mob story. The best ones make the ocean feel like a character—vast, mysterious, and deadly.
I stumbled on one last year called 'The Finestra Syndicate' that was surprisingly gritty, focusing on illegal shark finning empires. The tension came from the marine biologist protagonist going undercover, and you're never sure if the betrayal will come from the criminals or a sudden great white.