Personally, as a running coach and as a runner, I believe that putting a swim day in after your hard effort days is probably magical. I know for me it is helpful. Recovery days are just that, a day to allow the magic of your hard training days to settle in, the body to recover and process the effort and damage/inflamation you have done and make the adaptations that will make you stronger your next outing. Give swimming a try and you be the judge. Look at my other blog post about swim equivalent miles if you are really worried about mileage. I really believe a good swim post high intensity interval sessions is much better than "junk" recovery miles!
Jeff Bangle, DVM prrunningcoach.com
Int J Sports Med. 2010 Jan;31(1):26-30. doi: 10.1055/s-0029-1239498. Epub 2009 Nov 11.