Haven't had the pleasure of sailing on the Coral....yet. We have, however, done many cruises in October and have been very lucky with the weather. Although, it is always impossible to predict since it is still in hurricane season. We had booked the Island for the 10 RT Panama Canal for next May, but had to postpone due to finances. Hopefully f-mattox will see your post, I believe he's sailed on her. Best wishes!
According to my Top Secret resource, October is the worst month to go for possibility of rain. With the average monthly rainfall being between 4.1 - 5.2 inches. Average temperature in October is 83 degrees.
Then I say go for it! A Panama Canal cruise isn't something that everyone has the time, money, and opportunity to do. So why not take advantage of this great opportunity. No one has a crystal ball to predict what the weather will be on any given day. So don't obsess and worry about it. It is what it is. Just go and have a great time. You may luck out and it won't rain on the day your going through the canal. But if it does, be prepared with your umbrella and long sleeve sweater or windbreaker. You can always watch from your balcony or a public area like the Horizon court. Just go with the flow and enjoy.
I have to second Cruise Fanatic's thoughts: rain is a strong probability anytime at the Canal. Like I said in another thread, I've been twice in April and got rain both times; but not all day downpours--just afternoon thundershowers, like so much of that area. I wouldn't even worry about the weather--go when it's most convenient for you.