Water Tactics?

[Gargoyle]

Okay, you've taken half the map and eliminated all but 2-3 of the other pretenders. You have a large Land army, high income of gold and gems (but low water gem income). You have no water troops or commanders. One of your opponents (Ulm) has invested greatly in troops that are waterborne and has several water provinces in his control and has annoyingly assaulted you from them, forcing you to set up a fantasy version of Normandy coastal defenses.

If you do not naturely have water mages or areas that can generate waterborne troops and commanders, how would you counter waterborne activity by your opponent?

Is there a solid way to self-start a waterborne strategy late in the game under these conditions?