If I recall correctly we have only seen one of the Iron Islands in the show - Pyke. Maybe some of the other islands are covered in trees?
As for the time that passed between to two battles at sea. It must have been some time as Bran and Meera covered the 600 miles from Castle Black to Winterfell and Olenna made it from Dragonstone to Highgarden. Plus Grey Worm himself had the time to sail from Dragonstone to Casterly Rock.
Yeah, I think this is the way we have to look at it.