Mamie Gummer, who made a well-received Off Broadway debut earlier this season in “Mr. Marmalade,” has joined the cast of Theresa Rebeck’s “The Water’s Edge,” set to begin perfs at Second Stage in May.Thesp, who is Meryl Streep’s daughter, joins Kate Burton (“Grey’s Anatomy”) and Tony Goldwyn (“Manic Flight Reaction”) in a show about a woman and her children confronted by her estranged ex. Austin Lysy and Katharine Powell round out the cast. Another Rebeck play, “The Scene,” won plaudits at Humana this year, and Rebeck has said that show is on the table for Second Stage’s next season, which has not been announced. Will Frears directs “Water’s Edge,” which opens June 15.