The Midmar Mile is a 1600m open water swim in Midmar Dam. As this is not set in a pool, weather can play a big role on the day in water conditions. ie. smooth or choppy. As there is no black line to follow on the bottom of the dam to ensure you swim as straight and on route as possible it is important to focus on a large object on the side of the dam (tree) near the finishing point. Every few strokes you can lift your head and access where you are according to the landmark.
My advice is to be fully prepared and confident in your preparation. For a comfortable race day, training required is at least 3 x per week, from 6 weeks before race day. Sessions should start from around 500m depending on your base and build up to at least 1800m. I also suggest a few sea/dam swims closer to race day to guarantee an enjoyable day at the Midmar Mile.