I once fell in the water during winter - the water must have been around 0 degrees celcius (32 degrees fahrenheit) or less, as the water froze that same week. I kid you not, it took only seconds before I felt that all strength had left my body - very different from controlled training situations, where you know someone is there to pick you up. However I'd say that 63 degrees fahrenheit (17 degrees celcius) is a different story if you can maintain the heat exposure lessening position (HELP). She couldn't as she wasn't wearing a PFD - glad she survived!
BTW. The temperature of the sea around here is around 10 degrees Celcius right now, probably less offshore.