Layers....you will probably need a fleece jacket for north of Mexico but in Mexico it will be warm to hot depending on the ports.
We did the MR in mid Oct last year ..it was a bit rocky from Catalina to Cabo but not too bad.
We left from San Fran
Alexandr Pushkin, Carnival-Holiday, HAL-(old)Westerdam, Orient lines-Marco Polo, Royal Olympic-Olympic, NCL-Dream, Dawn-Princess, ACCL-Grande Caribe, Oceania-Insignia (3), Regatta(4) Marina (1) St Lawrence Cruise Lines-Canadian Empress
Scenic River cruise June 11,2012