If they are your pet, put them in shallow water and give them rocks that go above the water to give them a place to bask and to rest. They will drown if the water is too deep. Consult the person you are buying them from to find an ideal water amount depending on your tank size.
Well...turtles can't swim in deep waters because they weigh too much. Use the website link, placed further down this page, under Sources and Related Links, for more information. That website gives lots of information about turtles. If the turtle shown on that link is not the one you want...type in the search box the name of the turtle you would like to learn about.
