Hi Andy,
Diving off Izu (mostly west Izu for me) and some of the Izu islands (Oshima, Shikinejima) according to my logbooks average min temps at depth are around:
Celsius: May 16 degC, Jun 22 deg, Jul 22, Aug 23, Sep 25, Oct 24, Nov 22, Dec 17
Fahrenheit: May 66, Jun 77, Jul 77, Aug 79, Sep 83...