Are you talking about all Box turtles or just the eastern box turtle? While eastern box turtles most commonly live in woodland areas in dry leaf litter, some of the other box turtles, such as the southeast asian box turtle, actually swim quite regularly. Southeast Asian Box Turtles live in swampy areas, and swim quite regularly. At the zoo I volunteer at, I work with two Box turtles, an eastern and a southeast asian. While the Eastern almost always is buried in the mulch, the Asian Box is seen in the water much more often, probably around 30-40% of the time. Even the eastern, while it prefers the mulch, I have seen in the Water on occasion.