Byron's Pool is between the villages of Grantchester and Trumpington, about three miles south of Cambridge. It is named after Lord Byron, who is said to have swum there. Certainly Rupert Brooke, Virginia Woolf and Ludwig Wittgenstein all swam naked there in the early twentieth century.
The site is now a small woodland with a circular walk of about one mile, popular with dog walkers. There is a free car park there, but for a longer outing the walk or cycle ride from Cambridge is highly recommended. You can go all the way along the River Cam across Grantchester Meadows. When you reach Grantchester, turn left along the main road, and Byron's Pool is a few hundred metres on your right. (Please be careful crossing this busy and windy road).
You may have some trouble with GPS reception at the cache site, especially in the summer, but if so you should be able to find it with the hint. Please make sure to hide the cache well afterwards, as it can get very exposed otherwise. Thank you.
While in Grantchester, you can also see Rupert Brooke's name on the war memorial in the churchyard, and do the caches Enigma: Mission 3 - Turing's Old School (GCJN63) and Peering Over Lord Archer (GC189FJ). Then you can stop in one of the pubs, or the (slightly overpriced but nice) Orchard tea gardens, for some refreshment before the return trip.
Information about Byron's Pool: http://www.orchard-grantchester.com/Grantchester_Group.html
Directions for walk from Cambridge: http://www.plcane.clara.net/cambridge/granchst.htm