It depends on when and where you go. In Possum Kingdom I have seen visibility range from 10' to almost 60'. If there has been a lot of rain then it is usually not as clear. I would say the average for the lakes I've been too is around 20'.
Cost wise it can be expensive upfront. To get certified you could probably figure $250. You will have to buy mask, fins, and snorkel. From there your equipment you can continue to rent or you can buy your own stuff. The good thing is that even though it can be expensive to buy everything with a little effort, TLC, and regular maintenance it will last for years. The equipment is like everything else, there are cheap ones and there are some really expensive ones.
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