Fateful by Claudia Gray

It's about a servant girl named Tess in 1912, who wants to escape from the house where she works, and most particularly the lecherous young lord of the manor. But that's not her biggest problem. While on a voyage to America, where she plans to escape and start a new life, she meets Alec, who's ruggedly handsome, fabulously wealthy, intelligent and yet so clearly troubled that she'd rather not fall for him, but she does. That also is not her biggest problem. Alec, it turns out, is a werewolf ... one cursed to change every night, unless and until he surrenders his independence and joins the Brotherhood, a pack of violently misogynistic werewolves who have been tracking him for months. You'd think that would be their biggest problem, wouldn't you?

But no. Their biggest problem -- though they don't know it yet -- is that they're aboard the RMS Titanic.