Mate you will be fine. If you can swim all is good. If you are worried, book in for some adult swim lessons, just 2-3 to improve your confidence/ability. I'm a good swimmer but I can tell you now it was easy. We were in the pool for about 2 hours, and there is a lot of treading water, but you're sorta moving around etc. That wasn't tested, if you needed to just mention that you're not the most confident swimmer around and hang by the side in the down time.
As for the actual test, its about 20 m across the pool, any stroke, can take as long as you like. at the other end you grab someone and drag them. No special technique, and people float pretty well - they're wearing lifevests. all you have to do is grab their collar, and swim back, slow as you like, it really isn't something to lose sleep over
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Hope this helps