AFAIK the water competency cert comes in when you study emergency procedures during training and it is a pass or fail component. They make you swim up the pool rescuing someone and from memory tread water for 5 minutes, all with a life jacket on. Perhaps it's changed, but I can't imagine it's that important.
So much has been said on these forums regarding recruitment days but just be yourself, smile heaps, address people by name, don't be overbearing, but do be confident, make eye contact, offer people water/tea/coffee if you're getting yourself one, don't say you're applying bc you have a passion for travelling and you want the discount travel, and if you make it to the panel interviews (if they still have them), then I find it useful to have a closing statement that will make them remember you.
Good Luck!