I often wondered this when I would find my self surfacing after scuba diving in Australia in the middle of a sudden thunderstorm. My guess is that there would be some localised heating at the point of impact probably boiling the water, though as resistance decreases as the area of crosssection of the resistor (the sea) increases , the current from the strike would decrease as it spreads out. The submerged diver/fish would be in serious trouble if near to the strike but danger would decrease with distance from the strike. A number of electric fish, rays eels etc. can discharge through sea water just by putting your hand near them, which in my book serves you right for prodding fish when underwater. As for being on the surface with your head sticking up above the waves I always considered this a good incentive for adding another 20 mins to the dive, and breathing a little slower.

Electric shock passing through their bodies certainly would be fatal to any wildlife and swimmers close to the stroke. A bolt struck a wet soccer pitch in South Africa, knocking down players tens of metres away. Some needed medical attention, possibly including CPR (unsure about the details). Assuming that the very wet ground approximated to water, I would not like to be within 100m of a marine stroke, either as a fish or as an uninsulated swimmer in deep seawater, and shallow water should be worse. To a good approximation the strength of the shock should be inversely proportionate to the square root of the distance from the stroke. If one could barely survive a stroke say 100 m away, then one would get 1/4 of a fatal or paralysing shock at 200 m. On such assumptions one would get only one hundredth of the shock at 1km, perhaps an admonitory tingle. Therefore oceanic lightning should be a statistically remote threat to wildlife. Personally, having experienced some 12V shocks on a sweaty body, and having heard of someone being killed by a hefty 12V shock, the merest rumour of thunder would get me out of seawater pretty smartly. But suit yourself!

Hasty but dry,

Jon