I visit this area often, and as you will see, it's a nice place to spend your lunch hour in Summer. Remember to bring some bread for the variety of bird life that settle around the water.
Well worth taking a look around the huge Decathlon warehouses as well while you're there. It's amazing what sport you suddenly want to get into!
The nearest tube station is "Canada Water" on the Jubilee or East London line. The cache itself should be easy enough to find as it's location is pretty obvious when you've found the area. The encrypted clues should really make it obvious though!
Good luck