Answer:
Well, the sea was obviously one of the main places to receive imports from other countries and send out exports to other countries. If the sea wasn't there, they wouldn't be able to receive what they did or send out what they did, therefore greatly negatively affecting the economy that they did have.
Also, they (I think) received currency of some sort for all their exports, so if not for the sea, the amount of money in the country would have been greatly depleted.