Still it will be not a peak season of swimming so facility on the beach can be not well-prepared or not available. Although you can avoid a crowd of people ;)
On 1st July 2013 at Amanohashidate beach. You can check further info on the following link.
http://seasidekyoto.blogspot.jp/2013/03/enjoy-swim-in-one-of-most-beautiful.html
On 2nd July 2013 at Tango-yura beach. You can check further info on the following link.
http://seasidekyoto.blogspot.jp/2013/03/15km-long-shallow-beach-in-tango-yura.html