The Tides of March by K. Sterling

Welcome to Pooles Island. Population: 1 very grumpy merrow.

That’s all that Anthony Costa knows about his destination. Weak and barely clinging to consciousness after being attacked by an undead changeling, Tony is left in the care of Ronan O’Sullivan. The young anthropology professor’s fate hangs by a thread and the hideous merrow is Tony’s last hope.
Scorned and abandoned, Ronan has no use for men until he finds a beautiful Roman castaway on his beach. Bound by an ancient oath, Ronan has no choice but to care for Tony. But being a merrow means that Ronan is also burdened by a strong desire to claim the human inhabiting his bed.
There’s more to Tony’s story than he ever imagined and Ronan finds out he’s more than an ugly face, but a mysterious foe has unleashed an ancient horror upon Pooles Island. Ronan sets Tony free in an attempt to fight it on his own and finds himself outmatched by a kraken. Will Tony and his friends make it back in time to save Ronan, or will their love story get swept away with the tide?