The answer to this largely depends on the sea state. But if the suit is dry, and the wearer has a life jacket on, their chances are better.
The suit has to satisfy what's called the ‘262’, meaning that in two degree Celsius water over a six hour time period, the wearer’s core temperature can experience no more than a two degree temperature drop.
The other key piece someone would need to survive a storm is a life jacket designed specifically to be worn with the immersion suit. It should have a soft plastic spray hood that could help keep water from splashing in your face.
But while both the suit and lifejacket are able to float, they are not necessarily life-saving flotations, especially at high sea. These suits and life jackets work great in calm sea conditions, in low waves, and in moderate seas — but in a storm, there's no absolute guarantee.