Probably worth posting this in the Saltwater section of the forum as this is slighty more advance to be in the beginner questions!
I've read of people doing this with no problems but you have to watch for pollutants and chemicals in the seawater. I maybe wrong but I believe moving sand from public beaches is illegal, again, the saltwater section will be able to advise you more on this matter.
In my opinion it makes more sense to make your own sea-water from RO Water and Commercial Salt. The reason for this is that you can be sure the water is pure and free from harmful bacteria.
I would suggest posting this same question in the saltwater section. There are some very knowleadgeable people over there that will certainly be much more help.
Hope this helps
TIM