Uh-oh. I just tested this and it seems that in MGE ship units cannot be used for trade even if they're flagged as 'trade' units in Rules.txt. When you try to trade with a ship, it says "Units such as Settlers, Caravans, and Diplomats, which have a combat strength of 0, are considered non-combat units. They may not initiate combat with enemy units." It seems that whatever parameter triggers this message overrides the designated unit behavior. The question is, can it be altered? If ships cannot be made into trade units, I will most likely have to disable trade in my scenario, which is a major bummer. Maybe you could look into this? Ofc it may be that in ToT this issue is already corrected; in this case ignore my ramblings.