The Jennifer Morgue: Book 2 in The Laundry Files by Charles Stross
SOME AGENTS HAVE ALL THE FUN. OTHERS SAVE THE WORLD.
Bob Howard is an IT expert and occasional field agent for the Laundry, the branch of Her Majesty's Secret Service that deals with occult threats.
Dressed (grudgingly) in a tux and sent to the Caribbean, he must infiltrate a millionaire's yacht in order to prevent him from violating a treaty that will bring down the wrath of an ancient underwater race upon humanity's head. Partnered with a gorgeous American agent who's actually a soul-sucking succubus from another dimension, Bob's mission (should he choose to accept it) is to stop the bad guys, avoid getting the girl, and survive - shaken, perhaps, but not stirred.