We're trying to say that some dinosaurs would have had the intelligence of birds, minimum. That means they would have possessed problem solving abilities and the mental capacity to construct tools. Now, dinosaurs also had far greater physical dexterity and strength than birds, not mention claws and limbs which are far more suitable to tool construction than, say, wings. All of this combined, I'd say it's easy to conclude that dinosaurs would have had far more ability to construct tools and build structures than birds do. And remember, this is a conservative assessment.
Now, let's say they were more intelligent than birds. Hardly a giant leap, especially now that more and more scientists believe they may have been warm blooded. What then? Why couldn't they have built simple rafts? Obviously, there would have to have been pressure to migrate in the first place, but that goes without saying. Now, over the kind of periods we're talking about (several million years), why wouldn't such dinosaurs get better at boat construction?