How to Optimize old meshes?

I am working with a very old mesh; actually it is a terrain. I selected a part of it, where the surface of water is.

Thanks in advance for taking a look at my issue~

Moved from “General Forums > Blender and CG Discussions” to “Support > Modeling”

Looks like a flat surface.

Just delete every internal edges, select the outer waterline and the island waterline, press ‘f’ = fill it with ngon (1 face) and redo the uv mapping.