You will need to wear a pocketed bathing suit or sport bra to ensure that the prosthesis(es) is/are secure on the chest wall and not take the risk of falling out. Mastectomy boutiques usually have these items however, if you do not have access to them you can always have a seamstress sew a pocket into a bathing suit as long as it does not compromise the shape and integrity of the prosthesis(es).