I agree with what the others have already said. Staying calm is so important. Plus learning how long you can tread water is also important. Go out on a small day and just swim and tread water. The more you know about your own abilities will help keep you calm when something does happen.
As for leashes, I'd stay away from FCS. They seem to break far to often.And check that your leash is tied on correctly. The best leash out there can't do anything for you if it comes untied. I actually use two strings just in case. Plus I can give one to someone else if they need one.