Our September Zenith sailing to Bermuda had photography lectures from Conde Naste Travel Magazine. Check the cruise line's own site and the information should be there. Not all sailings have a theme/lecture or celebrity gueats on board, but the site will tell you which ones do.
Just back from Constellation. They had several guest lectures on a variety of topics. Also had a naturalist come aboard for the day we cruise the Saguenay River for whale watching. She gave a running commentary & then a special lecture on whales. It was awesome.
We just returned from Hawaii on Infinity. They had an excellent speaker on Hawaii and its history. Dr. Jay Wolfe was a wonderful storyteller and his programs had to be moved to the theater because of the crowds. We didn't miss a day.