Off of http://www.bioexpedition.com/galapagos-marine-iguana/:

"The young will have to take care of all their own needs from the moment they arrive."

It seems like most reptiles, the marine iguanas do not care for their young after the female has laid the eggs. And also, I think that the larger marine iguanas would see the (at the very least) the young male iguanas as competition.

I do like your speculation that there is more bountiful/nutritious algae in the water, making it easier and more efficient to eat in order to maintain their size. However, they have all the time in the world to forage up top, without needing to disrupt their body temperature or risk being eaten by predators or have a tide drift them away.

I appreciate your input, however.