Hi Amonicane, nice to meet you!
As far as I know is quite simple changing a unit terrain restriction, for example, for your unique unit just change the number of terrain restriction (I assume it is 7) with the one used by Flag A (it is 0) so it will be now able to go aver land and water as well.
Now, the part I presume can't be done is, changing graphic accordingly to a terrain, it can't be done so, your unit will still walk/run on the water like Jesus done once!
Hope it helps and also hope you will find a way to change a unit graphic depending on terrain it is located!
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MY GAME VERSION:
Age of Empires II The Conquerors 1.0 without any patches, just the ES release in August 24th 2000, proud to playing this game version!