Bone explained to me that a unit in boat will not be damaged if the boat runs around waters.

If nothing works...all heros of Medieval Europe had a sickness/war injuries, and couldn't resist without a monk's assistance on long period of time

How do I tell the game the Hero(which has to be a specific unit type Eg; William Wallace) is on the water? When by default he will be in a Transport Ship?

I appreciate the swift feedback from everyone so far!

Thats not an entirely stupid idea Coat, only problem is if it proves to be a long game chances of it being forgotten about is high, and these heroes count as the players defeat condition.