If you lived in 'Tornado Alley' in America, amidst such storm-battered states as kansas, you will sometimes hear the warning to stay out of lakes and recreational pools when a thunder storm is approaching.
Lightning always seeks to 'earth' itself - or neutralise it's energy. So it will avoid water and aim for land where possible.
But in ocean covered areas this is impossible.
Water is a good conductor of electricity. And like all good conductors the energy travels on the surface. So when the lightning strikes a body of water it dissipates out over the surface of the water. This explains why fish aren't afraid of lightning - becuase being (mostly) under the water, the energy doesn't reach them. If they surfaced at the same time nearby to a strike though they would be affected.
The area it spreads is proportional to the intensity of the power. The more voltage from the source, the further it will spread with deadly affect.
I recall the case of a rain-soaked football field where a group of school children were playing football. A bolt of lightning hit nearby and the rain on the grass acted as a conductor. Many children were admitted to hospital with varying degrees of serious injury - dependant on their location on the pitch. Those closer to the point of impact were admitted with respiratory problems or heart failure. Slightly further away were those who were admitted with burns to their feet and legs. The parents watching from the sideline were unaffected as they were further from the shaft.