I disagree. I grew up around a lot of lakes and the rule was to get off the water as soon as lighting struck anywhere. Granted, it wasn't moving water and it wasn't inflatable boats. If the lighting is near, I still think the best place is off the water, in a low spot (not under something tall such as a tree or tall rock). That can be disconcerting as the low spot is often pretty exposed (think ditch).
The water may be the lowest place in general, but you are the tallest thing on the water! Now, it's also important to note that as a child we also weren't allowed to swim for a half hour after eating, so this may all be bunk!
No, I still operate by the lightening rule, but probably not as diligently as I should. It's a tough call I think, and either way the risks are pretty low (do you actually know anyone that's been hit by lighting?).
I am a river, babe - I've got plenty of time, I don't know where I'm going, I'm just following the lines..... - "We are water" by Shaye