According to the real Izu Islands, Oshima Machi, a small collection of villages on a part of the Izu Islands temperature in October is as follows:
Max: Avg: Min:
Temperature:
Max Temperature 26 °C 21 °C 16 °C
Mean Temperature 24 °C 18 °C 14 °C
Min Temperature 21 °C 15 °C 11 °C
Found here
http://www.tripadvisor.co.uk/Tourism...Vacations.html 
It doesn't seem very much out of the question.