Yoga and meditation work for me, not just for diving, but to help me stay healthy and relaxed in all aspects of my life. I also do tons of cardio and weight training.
I don't think it's a matter of having to " believe" in it. There is nothing that strasnge or mystical about stretching and concentrating on your mind and body. Doing yoga does not necessarily mean that you are attempting to hover up into the air with your legs crossed. I would say that it has more to do with exercising your body AND your mind. We are all full of 1000s of thoughts per hour, tracing off in different directions. Meditation helps to learn to focus your mind onto one thing or no things (among other things!). There's really nothing that mystical about it. Martial artists will tell you about the power of focus and concentration. Before you can achieve something, the idea starts in your mind. Without thinking, you reach your arm out to grab a glass of water. That still started in your mind. Now, if you want to do something that is difficult, say hitting a baseball out of the park, then you will be thinking of that before you get to the plate. If you can train your mind to focus on that objective without any distractors, you will be ahead of the game. Once the pitch is thrown, you don't think of anything anymore; you just hit the damn ball! Nobody thinks, as the ball comes towards them at 160 kmh,"I need to change the angle of the bat by 3 degrees". They just do it. THAT is the state of mind that I try to achieve through meditation. When I'm freediving for fun, I am in a deep trance state that comes naturally to me now. I'm still working on maintaining that state of mind for competitive diving, and it will come.
If you look at meditation as a form of exercise for your brain, it will not seem as strange and intimidating. It's just another form of training, if you like.
Cheers,
Erik Y.