Is it legal to dive without certification?

Yes, and its also stupid. While it is not illegal to dive without certification, no reputable dive center or club would allow someone to dive with them without first being certified to scuba dive. Most state parks that have diving will ask to see your certification before allowing you to dive in their parks. In Florida a lot of our Springs/Fresh Water Diving is located in state parks.

So I started this post by saying it was legal but stupid, so why is it stupid. It's not that diving is difficult, it just requires a specific set of skills that are necessary to enter a world that the human body is not naturally designed for. Scuba Diving is more than snorkeling with a portable air source. You must be trained in the amount of time you can stay underwater, how deep you can dive, how to share air with your buddy if you need to and how to safely come up from depth. None of this is hard, you just need to know the information.