heater question

a GOOD submersible heater will blow it's fuse if there is water inside it. But the generality is if you buy a reliable brand (Enhim, Visa-Therm) you are safe, they have pretty good seals in them. And yes, if indeed the heater breaks and there is water getting to the electronic parts of the heater (not the heat coils) there will be an electric shock. This is not all that common if you treat your heater correctly though, usually, at the most, what happens is the water gets in the heater and the heat coils get wrecked. One thing that is pretty important to remember is that when you are moving a heater, you should always unplug it. Once a heater is outside of the tank it corrects it's tempature to the outside environment, and thus heats up. once you put the glass tube back in the water, it will shatter becasue of the temp differance, even if it is only for a minute. Also a helpful reminder is to always put your hand in the tank the back of your hand first, that way if there is a current of electricity, your hand will automatically move AWAY from the water. But to answer your question fully, no they are not 100% safe, nothing is, but they are very relable, and do not have a tendancy to do anything too out of the blue.

I have yet to see a post about someone getting electricuted from a heater. I have seen a post about someone using a heater even though it had moisture inside. Most will advise you to install a GFI at the outlet you're using to power the aquaria, that way you're covered. You could ask your father about pricing an inline GFI cord. It's essentially a short extension cord with a GFI that way you don't have to put in a new outlet.
Or, you could unplug the heater every time you put your hand in the tank.

GFI

You can get an inexpensive plug-in GFI at Walmart for ~$10, and a GFI/surge protector/power strip/6' extension cord is only ~$12.

I have one or the other on all my tanks.

I have seen posts from people who were electrocuting their fish (fish curved sideways and floating) because of some motor in the tank shorting, I can't recall if it was a powerhead or a light fixture + water. I recall they said they got a pop of electricity when they put their hand in. I have seen posts in pond forums of bad equipment shocking both man and fish.

I have dropped my light fixture into the tank, one of the bulbs was slightly under water, and I grabbed it with out thinking about getting electrocuted -- I was thinking it would explode the glass, but it didn't, it didn't even pop the fuse or hurt the light in any way, amazingly enough. I slapped myself afterwards for not thinking before putting my hand in the water, though I was quite aware that the tank was on a GFI.

I need to remind myself from time to time that if I get hurt there will be no one to care for the tanks!