Yes, it is safe to defibrillate a victim who is lying on a metallic or wet surface. If the self-adhesive pads are applied correctly and provided there is no direct contact between the user and the victim when the shock is delivered, there is no direct pathway that electricity could take that would cause the user to experience shock.
If the victim is wet, their chest should be dried so that the self-adhesive AED pads will stick properly. As with any attempt at defibrillation, particular care should be taken to ensure that no one is touching the victim when a shock is delivered.