Well, there isn't any most of the sand in that part of the gulf is based on arongonite,"coral and algal bodies." I would be concerned about chemicals, pollutants etc from a beach. If you have access that might be different story just go out a ways before you collect sand. You should also run it through a fairly fine screen to removes some of the hitchhikers you might find.
hth
Max
p.s.
I'd make sure it's not against the law before I collected anything. I don't know if Coasta Rica would have any legal issues or not.