Submersible heaters can, and usually are fully submersed, but usually depending on the brand, submersible heaters can have their tops slightly out of the water making adjustments easier.
Old time hang on heater have a full level mark.
Old time hang on top heater, see the bracket that hangs on the rim of the aquarium
Here is a stealth submersible heater, the highest white area on it reads "minimum water level", indicating that the portion above that line can be out of the water. Otherwise it is fully submersible.