An Incident at Bloodtide — George C. Chesbro

The Hudson River shone like silver during their evening sail. Unfortunately, when Mongo and his brother Garth step ashore, a self-styled sorcerer named Sacra Silver is also around. An old boyfriend of Garth's wife, Mary, he is soon tossed out of the couple's home. But Mary believes in his powers and is terrified of the possibilities.

Then a series of hideous practical jokes start to disrupt Mongo's life. They get nastier and nastier, until a local riverkeeper and environmental cop is horribly hacked to death. Mongo discovers some secret corporate pollution of the Hudson, and wonders if there is a connection to Silver. A hell-for-leather solo raid on an oil tanker bound for the dark Atlantic is the only way to find out.