Boyfriend deploying/promise ring??

So my boyfriend is deploying for a year long Mediterranean cruise, and we won't be able to talk much at all. I want to get him something along the lines of a promise ring to tell him that I'm going to be faithful and wait for him to come home to me, but he isn't allowed to wear rings or jewelry because of safety reasons dealing with his job. Do you guys have any recommendations for something that could take the place of a ring? I genuinely have no clue what to give him, but I want him to have something to hold close or to remember me by when he's at sea.