I spent two days trying to figure out why my water surface was completely transparent... The problem is that when I imported my landscape, my river was located at Z = -2820.
After further testing, it seems the water depth is linked to the camera's position. Even if my water surface is below 0 but my camera is above 0, everything is fine, but as soon as my camera passes under the grid (0,0,0), my water surface becomes transparent...