A friend of mine was mixing sand for a golf coarse and found a five cm wide heavy white ball shaped object in the mix.
It seems to be solid white but has a very thin brownish stained coating that seems to be just on the surface. It seems to have been an almost perfect sphere except for one side is concave and some chips take out of the surface.
Yes, it is possible to find a pearl in a large load of seashore sand, but the size you mentioned seems unrealistic, especially since the shape is round. The only way to be sure would be to send it to a lab for testing.
Before investing in a lab, an experience jeweler should be able to confirm your suspicions by examining the object. Let us know what you find out and a photo would be interesting also.