Canals

[kerog]

Units with the construction ability should be able to create canals that water-based units can move through.

[KGB]

In that case you'd have to allow construction of bridges, roads etc. Possibly even new cities.

Not sure if the game needs that kind of complexity at the moment as how often would be used?

KGB

[tabanli]

You can have a relatively weak, possibly a 2 turn unit that has ability to carry Sea Creatures.