Behind the Mask by Megan Derr

Kasumi is an elite warrior, trained in skills that very little of the world even knows exists. The price for his skills is high, and they are not sold lightly. Given into the service of a famous scholar, Kasumi travels the world at his side, a secretive, deadly shadow, his only defining trait a mask that no one can remember in detail the very moment they have looked away…
When Kasumi and his master board a ship, Kasumi is certain he will be facing nothing more than a very long, very boring trip. But the companion his master is travelling with proves to be the most idiotic, most infuriating man that Kasumi has ever met, and the trip proves to be frustrating rather than boring.

Then everything goes wrong…