Imagine falling forwards with all of that weight on your back. Your body would experience a braking effect when it hit the water but the air tank wouldn't. That would hurt.
Thanks, Mortartube! That makes sense. I did have a one week scuba course many many moons ago but can't remember them ever explaining why I had to do it - anyway, I preferred using a snorkel as I didn't trust the tanks and haven't done scuba since!
Apart from the weight of the air tanks landing on you as you hit the water, there is a very good chance that your mask would be torn off - another reason to go in backwards.
Its just a lot more comfortable and easier to fall in backwards, your body makes a smaller"footprint" against the water, your mask does not get ripped off and when you hit the water you go down then bob up looking straight at the boat you just left. Its all in all a lot easier. Plus you cant see the big mutha of shark thats going to have you for lunch!!!!
Thanks very much, Andy-hughes and Eastern. Eastern, your shark comment was a great start to the morning! I really liked it - today should be a good day now!
As Andy said, the mask is the main problem. That's not so much the risk of its being torn off, but the possibility - as I know from personal experience - that it can be stove in by water pressure, if you leap in forwards!