ocean tiles

I'm in the process of creating a scenario and want to simulate how something like a navigable river becoming more shallow affects what kinds of ships are allowed to sail up into it. Think the reverse of oceans: A galleass or caravel can sail fully up the river, but not a destroyer or battleship...

You know how an enemy trireme can prevent opponents from working coast/ocean tiles within a 2 tile radius? What is the applicable radius for an embarked land unit? I used to know this, because I had a spy in an opponent's coastal capitol and noticed my embarked land units were preventing him...