ok, thanks to all your advises now i have a large terrain with lakes and rivers. Both terrain and water is build with gl_triangle_strip, and since triangles are quite large, borders between land and water dont look very nice ...
so i think i should use tessellation to subdivide land and water bordering triangles and then texturing properly in order to have a better looking "bordering area" ? perhaps with a new texture which contains a piece of land smoothly turning to sea ?
if this is not clear i can make a html page to explain me better .