A fair question, and my answer? I am 100% certain that zoos could successfully keep marine iguanas in captivity. They've already done it, with some zoos having kept their animals for years, excluding our smuggler friends. Today, we have better technology, better transportation, a better understanding of the natural habitat and history of the species, better ability to mimic abiotic factors, better ability to recreate diet - I'm absolutely sure it could be done. There would be some trial and error, but that's how we got a lot of species, including platypus (or gorilla, for that matter) to go from "impossible to keep alive" to manageable, or in the case of some species, thriving.
Another thing to consider. Marine iguanas, in their wild state, are subject to mass die-offs due to El Nino years. In some years up to 90% of the population may crash. When I was in the Galapagos I saw thousands of marine iguanas - but there were also parts of the trip where I was literally walking on a carpet of desiccated iguana carcasses. Knowing this, marine iguanas could be collected from the wild in a perfectly sustainable manner at the onset of El Nino years, secure in the knowledge that they were probably going to die anyway of starvation, and therefore their removal likely would not be detrimental at all to the wild population.
