We went mid June. The heaviest coat I needed, I bought in the first port. Basically a fleece lined raincoat. Most of the weather we encountered was in the 50's. But the week before they were seeing upper 70's. So I am sure you can see a wide range of temps. And yes it rains a lot. The area is considered to be rain forrest.
You shouldn't need shorts, the ship will be warm. So pack some sweaters and such and you will be fine.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007