This can be done by using large rocks to create a pool of water on one side. The water level should be low enough that their nostrils can reach it while standing on their hind legs. For turtles that are strong swimmers, the water level can be even higher. The depth of the water should never be lower than the top of its shell. If the water level were lower, the turtle may have a tough time getting around.
Turtles seem to enjoy a basking area.
You can construct an island out of plain aquarium gravel, but keep in mind it should be easy to clean. You will also need a heater for the water. The best kind to buy is a submersible heater. Heaters that hang on the side are not recommended because turtles can chew them and they are not safe. It is also recommended that your enclosure have a hood or screen cover so they can t climb out, but also so that you can mount a basking light.