Diamondback terrapins are only fully brackish water turtles on the planet. There are some other species of fresh water turtles that have adapted to living in low salt level brackish water areas. Brackish water is were fresh water meets the saltwater. A lot of this kind of water is found in large bays. Diamondback terrapins also live in areas with only salt water.
There are small island all around Florida that diamondback terrapins live on surrounded with all salt water. The Florida Keys is another example of this. They only access they have to fresh water is from the rain.