Taken by the Sea Prince, book 1: Taken by the Sea Prince
Marcy Jacks2016

Matthew Ramiro once saved a merman's life and spent almost every summer after with the prince from the sea, falling in love with him, all the time knowing he could never have him. One day a merman drags Matty into the ocean to drown him and he almost succeeds.

Aldis has been waiting eagerly for the day he could claim Matty as his mate and lover. Finding Matty dying in the water puts a damper on that, and Aldis has no choice but to do the most taboo, illegal thing to keep him alive—turn Matty into a mer.

Matty survives, but now has gills, and the man he's loved for his entire life suddenly can't keep his hands off him. The sex is wonderful, and Matty is more than eager to be taken, but someone out there still wants him dead, and when another attack happens, Aldis might just have to defend his lover with his life.