Fresh water fishin in Surat Thani

I will be in Surat Thani in December and was looking on Google Earth for fishing locations. I do know of two fishing lakes and quite a good spot on the river up past the station. I found a few large lakes and a couple of large damned reservoirs to the South probably within and hour's drive.

I take it that permission is required from the land owner to fish but generally what happens if it is a large body of water like a reservoir or river? If is it is accessible without going through somebody's back garden is it considered acceptable to fish or is it best to stick where you can see local people fishing?