When a bolt of electricity, such as a lightning bolt, hits a watery surface, the electricity can run to earth in a myriad of directions.
Because of this, electricity is conducted away over a hemispheroid shape which rapidly diffuses any frying power possessed by the original bolt.
Obviously, if a fish was directly hit by lightning, or close to the impact spot, it could be killed or injured.
However, a bolt has a temperature of several thousand degrees and could easily vaporise the water surrounding the impact point. This would create a subsurface shock wave that could rearrange the anatomy of a fish or deafen human divers over a far wider range, tens of meters.
If someone in a metal-hulled boat was close enough to feel the first effect they would be severely buffeted by the second. Besides which, metal hulls conduct electricity far better than water, so a lightning bolt would travel through the ship in preference to the water.
When lightning strikes, the best place to be is inside a conductor, such as a metal-hulled boat, or under the sea, assuming you are a fish.
Last century, the physicist Michael Faraday showed that there is no electric field within a conductor. He demonstrated this by climbing into a mesh cage and then striking artificial lightning all over it.
Everybody except Faraday was surprised when he climbed out of the cage unhurt.