For something as big as a 55g, you might want to consider a pair of heaters, one on each side. That way, if one does go out, you've already got the 'backup' in place.
You can also lower the wattage just a little bit if you get two heaters. So say rather than getting one 200w heater, get a pair of 150w heaters.
In addition to heaters being fully submersible, also look into heaters that are 'ok' when out of water. Basically, some heaters say they MUST stay submerged all the time. Others will say that it is ok for them to be 'on' out of water. I like the ones that can be out of water so that I don't have to worry about unplugging them while I do a
PWC.