If you jumped in going front first, it's hard to clear the boat without hitting the tank on the edge of the boat with all that gear on, atleast in my mind. Because rescue swimmers go in front first, they just aim their toes up (with flippers on) and hit the water with their heals first.

I suppose to prevent injury to their backs but just guessing

Answer removed in protest of AB staff's lack of action protecting the membership from the COAT gang and their skewing answers.

Because if they rolled forward they'd hit their head on the inside of the boat ;-) I believe the real reason they do this is that it prevents the tank or any of the hoses from getting caught on the boat anywhere. It prevents injury and possible equipment damage that could be a problem once they're already submerged.

Ever try to dive into the water with a facemask on? It HURTS. And, if you jump in feet-first, the mask and regulator can be ripped off your face. So, they roll backward with their hand over the mask (usually) and reulator, too keep them from being pushed TOO far off, their head breaking the water's surface tension, rather than the mask and/or regulator. NOTE: I am not a diver. The above answers are logical, to me.

Because If you fell forward you'd fall into the boat

To allow the tank to go in without exerting weight to the diver at diving.

I have dived and it is easier to roll back. It helps keep the mask in place, others stepping off and diving you need to hold the mask in place

You only roll off a boat backwards if that is the easiest method and the freeboard is small. Off bigger boats you would use a giant stride entry or a forward roll. The giant stride entry allows your feet to hit the water first and slow you down. A forward roll is usefull if you have a camera etc to protect.

Maybe it is more fun to do it that way? haha, I have no idea...just my guess. :P

because the oxygen tank is heavy, and if they went in head first that would be dangerous

There are a few entry methods that are used when entering the water from a boat. The two most commonly used entries are the Giant Stride and the Back Roll in. The Giant Stride entry is usually done from a platform at the back of the boat which is near or at water level. The diver places one hand so that four fingers of that hand are on their mask faceplate and the palm of that hand is against the regulator. The other hand is place accross their ribcage over their buoyancy control device. The hand accross the ribcage is also holding on to their instrument console (air gauges and dive computer. This is to ensure that the mask and regulator remain in placeupon contact with the water and to make sure that the buoyacy control device doesn't "ride up". I prefer the Back Roll entry because it doesn't involve having to make your way to the platform wearing full gear on a rocking boat. You usually just have to plant you butt on the side of the boat and, after you are sure your entry way is clear and have obtained permission from the boat crew, just roll back. You still keep the hands in the same positions as in the Giant stride entry. The scuba cylinder takes the brunt of the impact with the water rather than your back. A front roll off the boat is not advised at any time because once you enter the water the forward spin continues and you could end up rolling in a position directly under the boat. Front roll ins look cool in the movies because stunt men have to earn their keep.

In short, It's a safety and convenience issue.

So Paddy asks Murphy: "Why do scuba divers always fall backwards off their boats?" To which Murphy replies, "Well, if they fell forward they'd still be in the bloody' boat!"

Most sit on the side?