Besides being very meticulous about your packing list try not to over think things. Keep thing as simple as possible, without missing necessities.
Things to remember to do pre-cruise: buy "cancel any reason" trip insurance, discuss the voyage and ports to be visited with the toddler's pediatrician, verify direct w/ cruise line the requested port-a-crib is weight appropriate for toddler or request safety rails for the daybed, and research infant/toddler friendly activities, tours, or beaches for each port.
Things to take with you: plenty of disposable diapers, a few swim diapers (for port only), sealed no refrigeration needed baby formula or food (if not eating solids), a reclining quick-fold umbrella stroller, at least 1 comfort toy/security blanket, a play-mat or blanket w/ nylon on 1 side (for floor/deck play), several bottles/sippy cups, disposable bibs, hats w/ brims, age appropriate sunscreen, pedi-meds (OTC and prescribed), and extra changes of clothing (or extra funds to use laundry service).
Things to leave at home or the car: a jogger or big carriage stroller, your pack-n-play, and car/booster seat.
Also if not already in a cabin w/ balcony investigate upgrading to 1. That way if the toddler is still napping you &/or your SO can sit outside while they sleep.