The typical attire for guests of a beach wedding is casual. Look closely at the invitation to make sure it says "casual" or "beach casual." This means slacks or dress pants and a button-down shirt for men and a simple summer dress or pants and blouse for women.
Even with casual attire or a beach wedding, tank tops and jeans are rarely appropriate. The guest should consider the bride and groom and their preferences when deciding what to wear. Some beach weddings say "beach formal" on the invitation, which asks for more than simple dress. This includes a linen shirt and pants or a summer suit for men. Women can still wear a summer dress, but it should be slightly fancier.