This is an opinion, not scientific fact...

I would think that if you are directly under the boat, with the boat between you and the sun, the coloring on the bottom of the boat would not make a difference because of the shadow cast by the boat. On the other hand, if you are not in the direct line of the sun, the coloring would make a difference. How much of a difference is debateable, butt, I think that any little bit would help when it is the difference between being eaten or escaping with your life.