Its not.
Tesselation on water causes all sorts of artefacts you have to counter when you use proper gerstner functions.
A properly LODing mesh with a vert every 10cm at LOD0 will give you better performance and a much better look.
Ofc its 34 tris per meter or something similar. But you budget that into the general scene cost.
Sure, its not as easy as setting a multiplier, and you have to work on the mesh.
The trade off is you don’t have to bash your brain in against the shader to get that to work without artifacts.
The high cost of water isnt due to tessellation or to the mesh, but to the WPO function combined to the transparency. And when on top of that your shader supports vector fileds, distance to nearest surface, etc, thats the real cost bump of it…
I can see procedural meshes needing to actually be created correctly, but the computation time in the end - between setting a tessellation multiplier and building it with the same end result tris count - shouldn’t really change all that much…