Most turtle setups will not have the water up to the top of the aquarium since there will be basking areas, so the hanging types will not work properly. For turtles, the totally submerged types are recommended. They can be oriented horizontally and therefore completely surrounded by water even in tanks with only a few inches of water. Note that the use of a glass heater can be dangerous. If the glass breaks, both you and your turtle may be electrocuted.
Turtles can be vigorous - be sure to protect the heater from the turtle.
The operation of the under-gravel filter will circulate water around the heater. If there is not enough gravel to protect the heater, consider surrounding it with bricks or stones. Some brands have a heater guard that fits around the glass tube and provides some protection. There are several brands of heaters available. The Ebo Jager heater is considered to be among the best.