Make sure that the cruise line knows if you are traveling alone or with other people. Singles are often assigned to tables with others traveling by themselves and there are special events such as dances and entertainment geared exclusively for singles and seniors.
Check beforehand that your chosen cruise line has any medical equipment that you might need already on board. Not all liners will have complete wheelchair access or provisions for bed extensions or shower assistance, so make sure that these are available if you need them.
Have everything that you need in your luggage. Most cruises have on board shopping but simple hygiene items can be very expensive and specialized lotions or medical prescriptions won't be available.
Dress for every occasion. Not all cruise lines have dress codes but formal attire is often expected at luxury dinners or late night dances. Depending on whether you are cruising in Alaska or Mexico, your clothes will be very different, but try to anticipate all the possible climates prior to your departure and be prepared.
Take advantage of the wide range or activities available. A cruise is the perfect time to try a new water sport or card game and most activities will already be included in the package. Games and shows are also a great way to meet other seniors and like-minded people.