I randomly encountered this place while doing a Hash House Harriers run, we were doing a trek right beside it and as we peered over the fence I was like "holy crap! What is that?"
This man-made hole in the ground (I don't know how or why) is filled with spring-fed clean water and is surrounded by a bunch of extreme-sport activities such as ziplines and my personal favorite - simply jumping off the side.
They have loads of food and drinks and if you want to get wasted, nobody is gonna stop you. However, I would be a little bit careful because obviously this place introduces a lot of opportunities to injure yourself or die. You have to sign a waiver upon entering the park that releases the owners from any sort of liability if you hurt yourself. Don't let that scare you though, because all the designated "drop zones" have been properly tested and yes... the water is as deep as it needs to be. It is actually so deep that without dive gear, you are not going to find the bottom.
In recent years they installed a giant inflatable section to the park as well and when I was there at least, there was a ton of lifeguards and wearing a life vest is mandatory for anyone under a certain age. There is also an area for the really young kids where the water is only about a foot deep that I suppose had to be constructed out of something that floats since this "lake" is hundreds of feet deep.
I can't remember the last time I had this much fun. It really brought me back to my youth and I got to run around with my friends in their 30s 40's and 50's and all of us got to be young again. At the steep cost of 100 Baht a person (around $3) there really isn't much reason to not go here.
TL;DR
This might be one of the coolest things to do while visiting Chiang Mai - I mean, who doesn't love a water park? The activities that they have cater to a wide range of kids and adults and despite the fact that they really could "Disneyworld" you on the food and drink prices - they don't do that. The price for everything is extremely low. I would consider this to be a "must do" while visiting Chiang Mai and at $3 a person, there isn't much reason not to.