You forgot sand dolphins, the lesser known close cousin of sand sharks.
In all seriousness I think a simple naval system would be cool and fairly simple to add. Just add the ability for AI's to use ships, the ability for ships to attack each other and have the armies duke it out on a new ship tactical battlefield. Then the devs or even modders could add sea resources(that require a harbor), and sea monsters and pirates that simply board your ship and start a tactical battle with the transports contents.
Ya dedicated sea units and combat ships would be cool, but my idea would be fairly easy to implement.