We just got back from 7-night West Coast today. I can't help you with water temp in Cabo because our kayak/snorkel tour was cancelled due to high wind and waves. I heard somewhere that the water temp was about 74, but not sure if that's right.
The water in Mazatlan was wonderful though. We went to Stone Island and spent the day on the beach. Jellyfish weren't too bad--we saw a few on the beach and I heard there were a couple of stings, but I didn't really see them in the water and no one in our group got stung.
Also, the last day at sea (Friday) was freezing all day long. We couldn't even really go up on deck for more than a few minutes with the wind and overcast skies. The first day at sea was cool, too, but not as bad as the last one. Bring a sweater or light jacket. Weather in Mexico was hot and humid, but with enough of a breeze to be okay.
Finally, if you can do it, a highlight (for me, anyway) was getting up really early on Tuesday (I think??) and watching the ship pull into Puerto Vallarta from Topsiders. PV is beautiful before the sun rises, with the lights and hills. (Not so great looking in the daylight though. JMO.)
Have fun!