" >and the water should be made out of a mesh which doesn't >have polygons
I don't understand how is possible .... "
Doesn't have polygons THAT EXTEND UNDER THE TERRAIN.
In other words:
Make a mesh that is a grid, instead of a plane.
Then, delete the polygons which are completely under the terrain.
Don't ask me how to determine which polygons are under the terrain. :-)
It may be easier to use the height data from the terrain to make a second terrain grid mesh that is flat but only contains those grid squares which have a vertex which is below water level.
I don't understand how is possible .... "
Doesn't have polygons THAT EXTEND UNDER THE TERRAIN.
In other words:
Make a mesh that is a grid, instead of a plane.
Then, delete the polygons which are completely under the terrain.
Don't ask me how to determine which polygons are under the terrain. :-)
It may be easier to use the height data from the terrain to make a second terrain grid mesh that is flat but only contains those grid squares which have a vertex which is below water level.