Just be aware that Gladhouse Reservoir has lots of large loose rocks on the shore. You might need to wear swim shoes to protect your feet. The small island is home to wild birds and so you mustn't land on it. You can swim over to the main island but aren't permitted to venture on to it. Watch out for fishermen in boats -- but the water is so huge that they tend to stay well clear of the busy swimming areas. If swimming at night remember to place some lamps on the bank so you know where to swim back to. Mobile phone signals are vary patchy in this area too.
Also, be aware that there can be lots of algae during the hot summer months.... it can be fatal to dogs and can cause illness in humans so best avoiding the place if you notice any.