As long as there is a discrepancy between the photo scenery resolution and the terrain resolution, triangles of water are inevitable. By increasing TMVL to 21, i.e. setting the terrain to its maximum of 19m, you are bringing the discrepency to the minimum possible in FS2002/FS2004, hence reducing the triangles to their minimum. By decreasing TMVL to 19, i.e. setting the terrain to a maximum of 76m, you are increasing the potential height and width of the triangles by a factor of 4, which really goes against what you say you are seeing, which is a little puzzling.