There was an Avis office very close to the dock. They picked us up at our hotel and dropped us off at the ship when we returned it - I assume they would also meet you at the dock. But maybe they even have an office right there - that would be good business idea.
Also, there are very few roads in Iceland - mainly on the perimeter and close to Reykjavik. We have driven in a lot of countries and I would rank Iceland as one of the easiest,
