Good stony beach for a firm entry into clean, clear water. Quite sheltered. Debris never accumulates in Barrow Bay but it does collect further south, around the peninsula, and so I always swim within or north of Barrow Bay (not round to Kettlewell). I recommend the southern end of Barrow Bay, just before the footpath reaches the small wooden footbridge. You only have to wade a couple of steps before being able to swim, and there are some mossy stones on which you can leave your clothes. Accessible by the regular 78 bus service from Keswick, the Keswick Launch, or the exquisite lake-shore footpath (there is a jetty and bus stop at Barrow Bay, also known as Ashness Gate). Barrow Bay is at the foot of Derwentwater Youth Hostel (a Georgian mansion with a waterfall in the grounds). After my swim I run up to the hostel for tea and homemade cakes - they always have a friendly welcome for non-residents, and you can hang your swimming things in the powerful drying room while you have your refreshments and admire the view.
This is a crowd-sourced map. Abilities vary, and conditions change. Always assess risks for yourself before getting in.
The lake-shore is usually impeccably clean, but I have sometimes seen little shards of glass amongst the stones.