For a casual beach wedding the girls could wear a cotton dress, about knee length and some sandals, sun glasses and a small clutch purse.  A light weight wrap for evening when the sun is going down, is a good idea for warmth.  Guys usually wear short sleeved cotton or linen shirts, worn on the outside or your pants  or tucked in, with cotton or linen pants, loafers, or casual nice sandals.   Guys should have a sweater or light jacket for the evening weather cool downs, too.  At some beach weddings the bride and groom offer their guests a chance to remove their shoes and wear flip flops to the ceremony.  They can pick up their shoes for the reception.  Everyone should wear a sun-block.