Nope. Roundup is not approved for aquatic use and is harmful to your amphibians in your pond. There are formulations of glyphosate that are approved for aquatic use, but even then I usually recommend them for plants around the shoreline. Reward or Weedtrine D are good multi purpose aquatic herbicide and algaecide for a beach, but if you can identify exactly the type of weed you want to kill, you can get a specific product for the task.