It wasn't always this way. Many centenalia ago, your planet used to boast a caste of ARISTOCRATS, whose duties included looking after the state of the oceans. During THE RENAISSANCE, however, these lineages were all but exterminated, viciously and without warning, by a self-proclaimed TYRANT who wished to seize the throne and rule with an IRON FIST as the only one of his kind.
Nowadays, the remnants of the old SEA HIERARCHY amount to barely 0.1 percent of the world's population — and that figure is dwindling as is, due to the general environmental collapse.
Most bodies of water have only further degraded with the passage of time, your whole species eventually gaining an INSTILLED AVERSION to any and all forms of it. There was a lawless period in the history of your planet where all manner of refuse — ranging from regular garbage to toxic waste and chemical mixtures — was unceremoniously dumped into the sea in the name of progress, using it as some sort of dross receptacle.
As a result, you have to force your lusus to stay away from her natural habitat, and with no way to keep her mind from the call of the wild, she's become HORRIBLY STIR CRAZY.
Better this than the alternative. This cocktail of hazardous substances has mutated harmless sea creatures into MONSTROUS CANNIBALISTIC CARNIVORES that feast on the flesh of their own. Unholy beasts with EXTRA EYES and WIGGLING APPENDAGES lurk in these brackish waters, waiting to strike at the slightest opportunity.
One man, a tempestuous bout of assisted violence, and sweeps later, the world still has open wounds to show for it.
Which brings you back to people like you. With the overpopulation of LOWBLOODS running amok, someone has to thin the numbers out. Or so the creed goes.