They didn’t use longships for trading either - they used a knarr - http://www.cdli.ca/CITE/v_knarr.htm
The longships were warships and raiders designed to move quickly and slip over port-protecting sandbars without the need for a local pilot - I think they only drew about 3 feet of water which isn;t much considering how long they were.
When I was in Oslo, we visited the Hjemkomst (I think that was the name), a viking ship that came from Minnesota (I think!!!), all the way down to Oslo. It had about two feet of space from the sides of the ship to the ocean. I cannot even imagine the courage it took to make it all that way.