Wow that sounds scary! The encounter with sharks is a privilege and something fantastic. All these techniques in this thread are for encounters with a big shark looking aggressive. Then yes, I would say go to the ground or wall and move slowly out of the area following the wall or the ground. In the blue take arms and legs together and stay immobile. In more than 3800 dives I never encountered something like that. All sharks I meet were scared of me. I had to behave in order to approach them. Some just ignored me but never one aggressed me... they are just like other fishes. Forget about this idea that it is a killer machine that attacks everything moving. If humans would be on the menu list of sharks it would be impossible to enter the water even for swimming! Also I never heard about an aggressive attack of a shark to defend himself! He just swims away!
I love to see sharks while diving and I saw some bigger than me and I am still alive.
Mako