I can't remember the thread here, but this topic was debated. I personally woudn't use beach sand as it may contail unwanted toxins, or chemicals. You may also introduce harmful bacteria as well. From what I understand, if you want "cleaner" sand, than you have to go out a bit further. It would seem logical to use the seawater as well, but I have also heard that you might have to do water changes more frequently, which would mean more seawater. I prefer synthetic as it has the necessary buffers for a healthy system.
For the price of sanitized optimal sand, I would go that route rather than chance it. Just my opinion.