We did this in May 2005 and as the area of the canal is a rain forest we did see a lot of showers, but the canal lock areas are fascinating. We passed under the Millennium bridge on our way to Panama City and the locks at the Pacific end of the canal. If a ship passes you it almost as you can reach out and touch those on it. Not many got off at Panama City is there had been numerous problems, not sure what it is like now but our trip finished up in San Francisco but we did go and see the divers in Alcupulco. We did our cruise on the Infinity so can't help with comments on the Oceana but have a look at the cruise reviews I am sure there will be some which might be helpful.
We have been on oceana 3 times and it is a lovely ship, the layout is easy and the antrium has a wow factor. The food was first class and the staff are very helpful. We done the panama in 2006 on arcadia we flew to Acalpuco and then went through the canal it was amazing we are looking at doing it again in 2010 as we have already 2 other cruises booked one on ventura nad the other on oceana. Sure you will have a great time .