Lock sea and ocean trading behind different techs

When you sign a treaty to trade, you immediately have access to trade with the other side of the world. The trade ships are super advanced whilst a war ship sinks to the bottom of the ocean if it stays there longer than one turn. 

My proposal, lock sea trading until Sailing is researched. The buying party needs to have the tech because they are sending the ships with money and get goods back. 

Additionally, lock ocean trading until Seafaring Mastery is researched. This makes it so that when Seafaring Mastery is researched, the world opens up a lot more and you can trade with the 'new world'

This also makes it so that land routes are more common.