The Pentland Hills Regional Park suggest wearing a bright hat and tow-float. https://www.pentlandhills.org/safety/safety-reservoirs
As mentioned above avoid swimming here when windy speed above 20mph. Also, be aware that very occasionally there may be a northerly breeze that may make it difficult to swim back to the start. If that happens follow the shoreline back.
The Reservoir is almost full with water during the winter months and so trees offer some shelter whilst entering from the bank. However, during the the dry summer months you can end up walking for 5 to 15m over pebbles and stones
before even getting close and therefore swim shoes may be needed. At that point you may also be seen by people on a path 130m away.. but luckily the path isn't too busy in the evenings. Horseflies are a problem on really hot evenings so avoid staying around too late as you may get a nasty bite that won't go away for weeks!
As mentioned above Glencorse Reservoir is used by paying anglers from March to October it is best to swim on Mondays when the fishery is closed or after 4:30pm on other days. But if you are entering the water when the anglers are there please keep a safe distance of at least 50m away from them