I can't understand how anyone about to watch this movie, knowing it stars Donald O'Connor, can be surprised to discover that it's funny, lighthearted, and contains some dancing.
I was thoroughly delighted each time I watched it. Besides the fantastic, under-appreciated Donald O'Connor, the cast includes Will Geer (Grandpa Walton), as well as many other actors fans of classic movies and TV will recognize, by sight if not by name. The plot is no more far-fetched than any other pirate movie, and the acting is very good.
If you want to watch a serious pirate movie, by all means put in "Captain Blood." But if you're looking for some upbeat entertainment, I heartily recommend "Double Crossbones."