Electricity takes the path of least resistance, in other words, lighting will hit the highest point! When you are in the water, your head will be the highest point, so if lighting were to strike the location you are swimming in, it will hit your head and trickle downwards. Also, water contains electrical currents, which means it will cause an intense shock when the two come into effect, in other words it will most likely be a fatal strike if a strike were to occur when your in the water.
How lucky were you to still have him? I would need to know where exactly the storm was when he swam! If the storm was miles away, then he wasn't in any true danger. However, if the storm was within a mile or right overhead, he was a brain dead idiot and very lucky to not have been killed.