Why do we need a safety buoy in training?

The sea is calm and warm. No jelly fish floating around, the pair of swimrunners is competent and confident, the route stays close to the shore, no wild off-shore crossing, no strong current, the two friends jump into the water with serenity. However, they haven’t done everything they could to stay safe : they are not carrying a safety buoy. You may ask why would they need to bother with this …