Leave them in for the winter if you don't get hard frosts. Cut them down & mulch them just in case. You can dig & divide in spring. They shouldn't need any more watering than they'll get from the rain.

â«It never rains in California...â« I'd leave them if I could, believe me. You also might try contacting the nearest dahlia society or association and get their advice based on your climate. Ours is going to be foul, wet, and windy this week and I am completely zone envious.

Here is a link that might be useful: California Dahlia Societies

dig if you want to. Your weather is ok for leaving in the ground, do not water, extra mulch does not hurt. You can dig and divide in the spring or just leave them alone and they will be fine,no water until they start to come up in the spring.