Criminals? Sailors? Part of some group?
In some cultures, tattoos are supposed to have magical powers, to give luck and good fortune to the one who has the tattoo, or even to heal sicknesses. Tattoos have also been a way to show ones status in the village/tribe/etc
And I know quite a lot of people who have tattoos and haven't even got so much as a ticket for speeding, like my mother has a tattoo on her leg, and she plans to get way more to cover up some scars, a couple of friend's of mine have some tattoos and I even have a monk friend whose back is covered in tattoos of angels.
All I'm saying is that not every single person who gets a tattoo is a criminal of some sort. Some of them just want to be unique, or have cultural reasons.