Not for Americans it isn't.
Both of my cruises have been out of the Med. First one was Adriatic in the month of July second was western in the month of March and April though the route is mostly similar with certain ports here and there altered throughout the season. Not one American was going to there for the beaches. The beaches are very rocky filled not sand filled by in large, you have to go to specific destinations within Europe for the beaches and most cruises are not hitting those. Boring is subjective, but in warm climates in the U.S. the beaches here are mostly sand beaches. I would never travel 8+hours (from the coast that doesn't include the flight to get to the coast) to Europe to go sit on a beach, it's not a good usage of my money and time even if I went to the Canary Islands or the Azores I would do a lot of historical site seeing with beach time mixed in. But it is to see historical sites and that is the travel patterns for Americans, the excursions don't lie either in numbers based on nationality.
If they want a beach they will normally go to somewhere closer, consider the Azores and Canary islands and south of France being akin to the Caribbean area in terms of proximity. We go the Caribbean if we want beaches outside the U.S. we do not go to Europe to do that, summer or otherwise.