i just came off a six month panama canal run (as staff member).
the seas are pretty calm, with the exception of the first and last sea days, and entering and exiting the panama canal at night (they had to cancel a show in the theatre once due to the motion, as the dancers were falling everywhere).
the climate in galveston is hit or miss. some days it is warm and sunny, others its not so welcoming. but after that, it's basically just hot. from cozumel to jamaica it was beautiful all season (only a few days of rain).
some things to bring: bug spray and maybe a plastic poncho if you plan on doing any excursions in costa rica (the jungles can get wet sometimes, and the bugs might be a discomfort if you arent prepared). if you want to do the "dunn's river falls" excursion in jamaica (highly recommended), bring some footwear suitable for water. and a hat that will provide good shade, as the sun can be intense in the caymans and jamaica. be sure to bring some bottled water out with you for your tour excursions as well, as in these ports, it's less available than most, and tends to be overpriced.
also, if going out in cozumel, plan your day accordingly so that you don't have to wait in the long tender lines. the local tenders were not in operaion during the last few cruises, leaving the ship to use their own. this resulted in very long waits towards the end of the day (and the ship having to sail late). most passengers seemed to wait till the last possible minute to get back to the ship, and were stuck waiting in line (the ship tenders have a 150 person capacity, and it's a ten minute trip by the time you get on the ship).
if you have any other questions, i'd be glad to help