I had to don a gumby suit in deep water as part of the requirements for my Merchant marine license. It was the kind they use in the North Sea in the oil and gas industry. You definitely won't sink with one on.They are difficult to put on because they float so high in the water. They do offer some protection against cold water and increase your survival time. How much I don't know. I believe it would be way better than wet clothes and nothing else.
