Anytime is the best time to go cruising, there is so much choice, so don't put it off. Book now.
Depends where you want to go and what you want to see. New England/Canada is best in Sept/Oct for example. Decide where you want to go and whether you want it hot, warm or cooler. Doing Pompeii for example in August is too hot for comfort and too many tourists but you might be happy with that. Personal choice.