Air is an electrical insulator. Electricity does not normally flow through it. During a lightning storm, the very large voltage between the cloud and the ground causes the air in between to break down along a very narrow path, and all the electricity passes down this path (called the lightning streak). When someone happens to be in this path, the enormous current that passes through the body kills him or her.
Sea water, conducts electricity well. As soon as the current enters it, it spreads out in all directions. Any fish in the way would probably experience a small current passing through it. It will give only a mild electrical shock which the fish can withstand. Fish that are very near the water's surface would be killed. But a fish below the surface would probably be quite safe.