We sailed with Oceania on the luxurious Regatta last year (14 nights, ended on Oct 8th) on Med Tapestry. Weather was such that we sunbathed in a Cabana on 4th Oct, but weather was just starting to turn. Warm enough, one wet day only. It was our first ever cruise and we had a superb holiday. Food was too good(!) and staff excellent. Go for it.
We have been to the med in Nov and it has been OK. You may get the odd bad day like anywhere else but most of the time will be sunny and warm.
I live in the Med and we do get overcast weather in October but if you live in Perth Scotland I can assure you it will be far better than there (I lived 15 miles from Perth for 7 years) and our poor weather would constitute a great summer in Perth Enjoy Im sure you will.
We went on the oceana last year on 4th oct our first cruise the the weather was not very good . we didn,t get into ibiza because of it and part of the pier in gibralta was damaged but we did get in there
other passengers who had done it before at this time of year said it was unusual so its just your luck.
we did enjoy everything else the food and entertainment were great.