According to a recent Harris Poll, nearly 30 percent of Americans have at least one tattoo. And of those who have one, close to 70 percent have two or more. More and more Americans have been getting them, and their popularity is strongest among Millennials and Gen Xers. But humans have been marking themselves in this way for thousands of years. An exhibit that celebrates the maritime aspect of this tradition opened in August at New Bedford’s Fishing Heritage Center. For this month’s Artscape: Nautical Tattoos, and the Stories Behind Them.