This may sound strange, but I wish we had purchased our embarkation photo on every cruise. Of our 20+ cruises, we have all but 2 photos -- I'm pretty sure we didn't buy them as we felt one or both of us did not photograph well. I certainly wish we had them now so that our collection would be complete.
On my first cruise I forgot my Norelco electric razor. So I had to use the plastic Bics and shaving cream, as a result my face bled allot.
I sleep in my white underwear T-Shirts. I forgot to pack them on my second cruise(no underwear for sale onboard nor in port). So, I had to sleep in my lightweight dress shirts . It felt unusual, but I had to get used to it.
On my first cruise, I should have spent money on those interesting wood carvings in the Straw Market in Nassau. Regetably, I fell for those pushy street vendors/hustlers and bought a bunch of junk T-Shirts and other crap. Next time I will polity say "No" and keep walking.
On my second cruise, I was in Cozumel. I regret not buying anything made of silver. They had good prices for silver there (and elsewhere in Mexico, I guess). Now, a year later, I am sure the prices on silver have gone up. Also, I should have bought my booze there. And I should have bought an extra bottle of Vanilla extract for one of my friends at home who does a lot of baking. The vanilla they sell in Mexico is the real stuff and allot better than the stuff you would find in your local supermarket.