system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes crystal clear water so clear? Does the water have the same refractive index as air? Is it a lack of sediment?
gpt
Crystal clear water is very filtered.  This process is done naturally, by rocks, glaciers, or pressure under the ground. Since the water is so clean, there are not enough ions for plants, bacteria to survive in well so they do not affect the clarity of the water. 

The water will obscure light more so than air, but you can see significantly far into the water.

Personal anecdote: In Iceland, you can snorkel / scuba in Silfra, which is some of the purest natural water on earth. you can see down at least 400 feet without issue. It was very fun and I'd highly recommend.