Actually, you haven't said where you live or where the tank is located. The wattage is based on the difference between the required tank temperature and the ambient temperature of the room that the tank is in. So long as the tank is in a warm room, 200W should be fine. Adding a second 200W for redundancy is a good move. If it's outside in an unheated garage then you might need 500W or more.
Most aquarium heaters are designed to be fully submerged, even the Ebo-Jagers that, to meet the more stringent Canadian safety laws, say not to fully submerge! It's a mad, mad world! LOL!