Swimming in the Lake District

It should be swimmable at that time.
Many many years ago I got inside the water at Lake Como (in July I think)
Beaches are a little pebbly or rocky. More than sandy I'd call the lake bottom muddy. Maybe some beaches may have brought in sand (small pebbles) from somewhere else, but not really inside the lake, just on the beaches. Near rocks and cliffs the water is likely clearer. Don't expect the Caribbean or Sardinia, though.

Pebbles versus sand versus rocks totally depends on where you are. Last September my husband (who loves swimming in lakes when we travel) swam in two places on Lago di Garda, at the southern end. Feltrinelli beach right in Desenzano was smooth light-colored pebbles. Jamaica Beach at Sirmione, at the very tip of the peninsula, was solid, smooth white rock (probably limestone). This made for nice clear water, but the wet rocks are very slippery! See photos here:

https://visitsirmione.com/en/what-to-see/nature/beaches/jamaica-beach/

We have both enjoyed swimming in Lago di Como, at Menaggio. This was in June and the water was cold; it should be much warmer in September. We were actually staying in Varenna but we did not swim there—the water was not clean enough (too many waterfowl). We took the ferry across to Menaggio and found nice clean water there. No beach—-the whole shoreline is lined with a concrete bulkhead to prevent erosion. But there were steps down into the water.

Thank you for the insights! We'll be staying in Stresa, Menaggio, and Malcesine, so we'll see where we might take a dip.