Maybe there should be a combination of lore and game mechanics to solve this. Allow any army to boatify, losing one season of movement.
From this point there is a wide range of gameplay mechanics and lore that can be used (not that any devs are reading this silly conversation):
1. There is a merchant marine guild that provides transport of goods and troops for all of the realm. They can be called at any time by a small magical coin given to the leader of every army. Instead of a coin it could be through use of carrier pigeons or however else we are giving orders in this game. For a price you can move any army to any location with total security.
2. There is a merchant marine guild that provides transport of goods and troops, but it has limited ships available to the highest bidder. There would be a ship per every x tiles of water. You can use the diplomacy tree to gain contact with these mariners. Once contact is made you may request transport at a given price. The default value would be set, but if say a nearby faction outbids you on the use of that boat, it will work for them on the next cycle of turns. There would be rules for acquiring ships, but once troops are on a ship, you would have complete control. Likewise, goods being transported by sea would continue until the end of that season. This would add a fun and competitive mechanic to seafaring not previously seen in any game. The point would be to either spend money and time ruling the seas, lending a new strategy to players that have little opportunity on their current continent. The issue of streamlining troop transport would be something to work through, but I think the idea has merit if they are looking for a full fledged seafaring system, without causing all the annoying problems found in Civ games.
3. Use the standard boat systems seen in Civ4.
4. Boats are created via magical reshaping of massive trees and a technological rediscovery of viking style, quickly manufactured boats made from trees and commonly found supplies gathered by your men in on their travels. These two methods can be researched. The first boating techs would lend to short travel (3 water tiles away from a coast). They can be made at any coast. Further research would allow greater waters to be traversed, possibly incurring some time penalties or requiring a harbor or drydock rather than being able to build it at any coastline.
5. "Shutup and play the game. The devs are busy with more important matters and seafaring is going to be minimal. If immersion is an issue, get drunk before playing." -Derek