I have put together a collection of mastectomy swimsuits that are ideal for ladies who have had breast surgery, and now wear a prosthesis. They are pretty, and have a higher neckline for additional modesty. They are all pocketed and will securely hold a breast from.
For swimming, either in chlorinated or salt water, I do recommend the use of a swim form. These are quick drying, lightweight prostheses that are highly chemically resistant.
I also have ranges of mastectomy bikinis and tankinis if you are interested please take a look.