Robbie Pennington: A big pool, maybe about 900m long. Deep along the north (Scottish) bank, progressively shallower towards the southern (English).
Easily accessed at NT 807 393 on the A698 between Birgham and Kelso, where there is some car parking and steps + jetty.
Update - the footing in the water adjacent to the jetty is pretty gooey but perfectly manageable. Immediately upstream of the jetty there is a very uneven shelf of rock under water for circa 15m, which is both difficult to swim over and walk on. Best return to the jetty is to stay about 20m clear of the Scottish shore until the jetty is abeam.
The pool makes a good start for a swim downstream to Wark, on the English side, though it's better to start at the top of the pool by Carham Hall. (Carham Hall Nursing Home was good enough to allow parking when we swam from there to Wark).