Chuck is right ... the first full day at sea can be kinda chilly (for us native Californians) -- about 65-68. But the next day at sea is wonderful. Same with that one day at sea going back.
We seem to always go to Mexican Riviera in October -- usually very good rates and weather is just fine (a little humid in PV, but it gets better as you go back up)
I also read here once that for every week in October that passes the weather improves and improves. I believe the tourist season begins in November.