Anything, even the smallest thing, has value to someone whose family barely eats or who may be living on the street. Most have moral values, though.

It is fairly common in Brazil to ask the person next to you to watch whatever you do bring to the beach while you go in for a swim, though. So choose to sit next to someone who looks trustworthy :-) or at least has as much stuff as you. Then they may ask the same of you, to watch their things. Guys have it easy, though, 'cause their swim trunks usually have pockets. (unless you dare to wear a sunga/speedo :-)

Also note that the water in Rio may still be fairly cold for swimming, even if the air temp is nice for sitting out in the sun in the afternoon.