I have seen lightning hit the middle of a sea lake. ( very happy I had not gone swimming).
The water did not boil enough to be observed at my distance, about 500 meters. No dead fish were washed out.
A boat or a head in the sea water will become a focus for the upward streamers that will join the downwards leaders and form a path for the energy of the lightning, and the consequences are the same as on ground for the victims. The good conductivity of the water will disperse the energy over some distance on the surface. I found some numbers on the web:
A typical lightning flash is about 300 million Volts and about 30,000 Amps. In comparison, household current is 120 Volts and 15 Amps. There is enough energy in a typical flash of lightning to light a 100-watt incandescent light bulb for about three months
The following sounds reasonable:
Apparently not, unlike air, the sea is a very good conductor as it contains salt. Rather than creating an direct narrow, path concentrating the "punch", the charge from the lightning strike spreads out sideways and downwards in an expanding half sphere from the surface. Any fish within a few meters of the strike area would probably be killed but beyond that they would probably just feel a tingle. Fish also tend to be a bit deeper in the water and not at the surface where the current is concentrated.
At the point of the strike the voltage potential is very high and electrons will dissipate away from that potential. The farther away you get from the strike the the less current there is to dissipate, it drops relatively quickly, it's the surface of a hemisphere equation so it's like a 1/r2 like a drop. As it dissipate the voltage will drop across this hemisphere. The voltage drop across any distance D to delta r, will determine if the current flowing through any objects in the water, and this my friend will determine if you shall survive or not in a lightning strike in the water.
Given these numbers (in my link) and a sea water resistivity of 0.2 ohm meters, you are only guaranteed death via lightning strike if it's closer than 6 meters. It seems that salt water is so conductive current would prefer to flow around you than through you.