FYI
Lightning can travel at least 10 miles from its source. Making yourself 2-3 feet shorter won't help much, and oh yeah, it is estimated that only about 3-5% of lightning injuries and deaths are the result of direct strikes. Just saying. There are really only three factors that affect where lightning will strike: Height of an object, isolation of an object from taller objects, and Pointiness. Metal objects do not attract lightning but are great conductors. (Auerbach, Wilderness Medicine).
The thing is, most metal objects are tall, isolated from other tall objects, and pointy. Look at a lightning rod. It's pointy, isolated, and the tallest thing around. It is made of metal so that once it is hit it will conduct the current through wires on the outside of the structure and into the ground (good example of this is the shack/cabin at Pitkin Lake outside of Vail...take a look if you are up there and you'll see how it works). Pipes and electrical wires in buildings do the exact same thing, which is why a building with grounded plumbing or electric is a safer place to be. The metal of a car also conducts the current around the outside of a vehicle and usually will jump from the bumper to the ground rather than try to travel through the tires to the ground (poor conductor).
Back to the river. So a boat on a lake with you in it, does attract lightning if it strikes nearby (isolation of object from taller object, but perhaps not on a river if it is not very wide). The rubber boat won't do much as it is probably wet seeing as you are in a thunderstorm. Still, lowest point, not in isolation as long as it is not a huge river, and not pointy. So all good so far; however, lightning is defined by its unpredictability and if you get hit, or your party is running tight and all gets hit, what happens. Blown off boat into water, knocked unconscious and fall into the water, etc.., etc.... So that is the risk you have to assume if you stay in the boat on the water. Obviously, there is also risk in stopping and spreading out on the shore ( in order to limit mass casualty in case of a strike) as you will most likely be closer to a taller object (bad) and might be closer to, or standing on, wet sand (also bad).
As with all endeavors, it is up to the individual to accept the level of risk and mitigate when possible. We could all stay home with triple locked doors (when it roars go indoors), but what fun would that be.