Using 3d objects to block off water - DarkRob's suggestion is a sensible and logical one, but has some challenges when put into practice. B)
I've tried a few combinations over the years but haven't found a satisfactory solution, other than falling back on the type 25 grey scale option so that the AI skirts around water, unless routed (Btw - the type 25 barrier doesn't appear to be applied correctly to the pond in the stock Quatre Bras map which perhaps explains why Reb had such problems when scripting).
Using 3D objects - SoWWL already has a square 3D waterplane object that can be placed when creating a map within Norbsoft's version of PowerRender. Their size can be adjusted but can be difficult to place satisfactorily when the ground level varies. Also, as ponds are usually rounded, square waterplane objects need to be sunk into the ground to look right.
Depending on a 3D object's properties, units will ignore them (collision switched off), move on top of them (bridges) or move around their edges (houses).
The pond in Quatre Bras uses one of these waterplane 3D objects.
I have tried replacing this with a version whose properties force the AI to walk around the edges. This appeared to stop units from walking across it, but made the units edge around the exact outline of the object, which looked odd and ruined immersion. Adjusting the collision radius made no difference. It also meant some troops scurried around like headless chickens for some minutes as the AI sought to find a way around. You sometimes see this when buildings in a town are close together and the AI seeks to squeeze through.
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You could put a further type 25 greyscale barrier outside of this, so that the troops avoid the sharp edges. But if you're doing that, then you might as well use the greyscale 25 option in the first place.
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One option would be to make a 3D waterplane object with rounded edges so that the interaction with troops isn't as stark, but suspect this would still look odd as AI controlled troops skirt around it.
Others may be more successful, but I've yet to find a satisfactory way to use 3D objects to block off water.