breath holding

Hello, When using an oximeter or a similar device to monitor oxygen levels during freediving, what is the percent/number that you should start to ascend for (ex. do I start ascending at 90% ,80% , etc.?) I know the precents can vary between beginners and advanced free divers so I want to know...

I'm new to diving, I've never really gone scuba diving but I've done dives where I just swim down and enjoy the water and the view. I was wondering what people like to take when others do this because I prefer to dive with no gear like in a swimming pool. Also wetsuits never really agreed with...

I am looking for some instruction-books, videos, etc.-that cover old fashioned snorkeling to depths of 30-50 feet. I just want to review techniques to stay down longer. Everything I find is on free diving. I am not interested in free diving. I just want to swim to a reef for a couple of minutes...