bear in mind that 4 gold per coast is ALMOST as good as a town would be at that point... but worse. so you're still better off working land tiles. I don't know how you can "use the sea to dominate". All cities are on land, so that's what matters.
If it's a multiplayer game you can play as the vikings, beeline astronomy, and hit them with berzerkers when they're not prepared yet. That's a fun gambit, but I don't think it would work well against the AI (it builds too many longbows).