What do you do if you need to use an aed on someone who has been submerged in water?

Drag them away from the water and dry them off as best as you can before proceeding.
If this is not done, there is risk of electrical shock for you and anyone around who maybe in contact with the water or puddle the victim is lying in.
(This is because water is an excellent conductor of electricity, and the AED is obviously using an electrical shock)