[No Spoilers?] What type of ships make up the Iron Fleet?

[Carl Drogo]

From reading the books I had always imagined the iron fleet to be made up of Viking-style open-decked longboats. I was quite surprised to see the Sea Bitch depicted as a regular galley of sorts.

I looked it up on AWOIAF, they have no description of the boats of the Ironmen, but show this image:

This would obviously support the longboat theory. Of course longboats make the most sense for raiders, because they can be beached.

Now did I just imagine this incorrectly, or were the ships actually described as longboats and they just changed it for the film? If so, why did they change it? I imagine the ship was CGI so they could just as well have done a longboat. For any "real" scenes a longboat would also have been much easier to construct. I don't get it.