Actually no. Even if you unload everything except one tile it will still freeze your editor.
It is the “landscape deform” part that causes the issue. If you try simple sculpting on this big landscape while Edit Layers Enable you will notice the sculpting halves your FPS as well. If you disable the layer edits your FPS is normal again while sculpting.
It is a “landscape operation” issue I believe, not so much water. You may experience the same on Landmass Plugin.