Just a point of reference. When I lived near the (Gulf) coast, sand near the waterline was rather like cement. It would hold a lot. I don't remember your sand being much different. When its saturated, it is hard. As for pressure - i would run about 20psi, but I'm a bit lighter than you. Really, your limits are having the tire fold over on the sidewall during a hard turn, and bottoming out (neither are typically a big concern on sand). You can go as low as you want if those things aren't an issue. (The CX gang goes below 20psi on 33mm tires at race speeds).