I know this is a bit unusual but I did some freshwater snorkelling in Australia and there's a beautiful world underwater in many rivers and lakes that no one sees!

 

I recently went to the sticky waterfalls and there was a couple of spots above and below the falls, but they were maybe a bit too shallow. Got me thinking that there must be other places with clear water, fish and plants? Perhaps near other waterfalls? I don't mind doing a bit of jungle trekking to find them.

 

Does anyone have any ideas where I could start looking?

 

And yes, I have thought of the moat/canal road/ping river, thank you

I think now difficult to find clear water, after very heavy rains and flush floodings.
When the currents are stronger, the water in rivers and lakes is more dark/brown.
Maybe you need to wait couple of weeks/months for it to settle down.
Sorry, I don't have any particular place in mind other than Grand Canyon, but that's too deep for snorkeling and probably you won't see much life there, you might find some dead people if you lucky though...
Some other places to consider - CMU lake, Huay Tung Tao lake, Mae Tang houseboats lake, Agriculture CMU lake, Green Lake hotel, Serene Lake in Mae Hia (very big fishes there).

Nearly every village/town in Thailand will have a dam for their water supply. No need to go trekking.