When we went with RCI we docked in Barcelona at 7.00a.m and our flight was not until early evening. Unfortunately you do have to leave the ship at the designated time as they need to get it ready for the ship leaving again that day. We were given a ticket the night before departure which advised us of our time to leave the boat as they do it in stages

Sorry but I cannot help you with with the storage of your cases though as we just took ours with us

You dock at 0600 but the ship has to be cleared by Customs before anyone can disembark, so allow another hour at least for that. Then you can have breakfast in the Windjammer and leave. If you want to be among the last to leave go to Guest Services the night before - they will give you a color coded luggage tag - when they call that color you can depart. You can probably check in at 10am at the airport and go back out again, but I don't think many airports have left luggage lockers any more. Check the airport web site and also the Bus station, I believe they do. http://www.venice-hotels.redflag.info/bus-station.htm
Not sure if the Train station has them. http://europeforvisitors.com/venice/articles/venice-railroad-station-faq-baggage.htm this article says yes, but I would check.

Thanks a lot guys.

Jim