The sound of waves sliding across the sand, children laughing, and people talking permeated Richmond’s Keller Beach on a warm Saturday afternoon in late August. Paddle boarders and swimmers dotted the ocean, with children splashing closer to shore. Seaweed dried on the beach, its scent mingling with that of food simmering on a grill. The water was warm and calm, said Christina Kossa, who lives in Berkeley and has been swimming in the bay for about eight years. It was…