I'm not too excited about the way sea travel is typically done.
I really don't like cities building harbors and then building boats and then having to load troops onto boats.
I also don't like units turning into boats to cross the water. There has to be a better way.
A. Flying shows us that the only thing necessary for water-walking to be interesting is stuff to do on the water.
B. Spells / Units / Creatures / Vehicles / Mounts / Items with water-walk.
C. The raise (and consequently lower) land spells could have their mana costs reduced to 5 or 10 (more terraforming = more fun anyways, especially with the beautiful updates on the cloth map).
D. Only armies with a water-walking unit that has the appropriate "vehicle size" trait for the amount of troops in said army could access waterways. The model for this unit would be used to represent the containing army, and it's movement points could supersede those of the other troops.
E. Certain water-walking units or vehicles would not be able to walk on land.
F. Beach tiles could act as gateways through the cliff wall that separates land and water.
F.1. Beach tiles could be limited to only functionally granting passage once brought under a player's ZOC, researched, and developed (higher level structures imparting larger modes of transit, and allowing for bigger armies).
F.2. Upon completed construction, a water-walking unit (or vehicle) could be produced, similar to the elementals created by shard shrines for Binders.
F.3. Beach tiles could be connected via a similar system as roads. *credit to Heavenfall for this idea
G. The lower land spell could be adjusted accordingly to change cliffs into beaches (which would still need to be brought under a player's ZOC, researched, and developed to be of any use).
H. Rivers could additionally be classified as water, allowing for interaction with directly adjacent objects via aquatic routes (the limitation of using the afore-mentioned beach tiles in order to actually move on or off land still standing).
I. Water could be further categorized into different types of tiles with varying effects, such as shallow, deep, calm, and stormy (perhaps even limiting certain size armies/vehicles to certain areas/depths, or influencing the route generation for "sea roads").
J. Units on water could require additional upkeep.
K. Any combination of the above at all.
(I'll keep thinking on my way to sleep.)