Diamondback Terrapins are not really in the same category as sea turtles. their life styles, body morphology, and requirements are essentially the same as pond turtles, like painted turtles or sliders. They do not have flippers like sea turtles, and are not adapted to permanent and continous life in the water. Sea turtle males never leave the water, and females only leave for a few hours each year to lay eggs. They have a difficult time on land. They are totally adapted to life in the ocean. They can and do travel thousands of miles in the open sea. They are tied to land only by the requirement that their eggs incubate on land. Sea turtles are unlike anything else in the turtle world. They don't really swim, they fly through the water like a bird in the air. They are incredibly fast and agile, wonderful to behold.
Terrapins have legs like ordinary freshwater turtles, not flippers, they do not swim nearly as well, and require dry land resting and basking areas, places where they can dry out and get some direct sun. They cannot be kept in a fish tank unless this dry land area is provided. Small specimens will quickly drown without shallow water and a secure landing area. Terrapins are really just freshwater turtles that have adapted to life in brackish water. They are found in bays, canals, and similar protected water near to land. They can't live in the open sea, and they don't belong in an aquarium either. In general, turtles do not mix well with other creatures in captivity.