There are basically four criteria by which I judge an Elvis movie: the songs and how they relate to the movie's story; Elvis' acting performance; Elvis' co-stars' performance; and the story itself.
The songs used in Clambake (especially the title song itself) are not well-written. In fact, they are so forgettable that I can't recall any of them as stand-outs. The best one is probably the one (title?) he sings on the beach. The song he co-sings while riding a motorbike in the movie is relatable to the story and it does look like Elvis is "into" the song. However, I think that speaks more to Elvis' charm as a musical performer. Although if you have ever seen Harum Scarum (and if you have not... I do NOT recommend you do), you will see Elvis at his most UNenthusiastic when it comes to singing the movie's songs. The song he sings to/with the children at the park playground is way too long and seemingly filler material... having little of anything to do with the story.
As for Elvis' acting performance, I've seen him give worse performances (Harum Scarum definitely tops the list of other Elvis movies I have seen), but I have also seen better (Loving You, Jailhouse Rock, Viva Las Vegas, to name just three). He definitely held his own in this movie. I'll leave it at that. I don't recall any fight scenes in this movie, which I find/found a refreshing change (it's been a few days since I watched it and have watched other Elvis movies since then).
Shelley Fabares (if you remember her from the 80s/90s TV show "Coach") was likable as the leading lady in this film. Bill Bixby (later most familiar as David Banner from "The Incredible Hulk") played the protagonist well because I didn't like him... which was intentional. Will Hutchins (maybe known from his earlier role in another Elvis movie, "Spinout") plays Tom Wilson, the waterski instructor whose identity Elvis' character (Scott Heyward) assumes in the movie... is adequate.
As for the story itself, it has (for me) two elements I have enjoyed in my own life (motorcycle riding and waterskiing), so I'm probably being more generous about this movie's appeal then I would be otherwise... but that is why all opinions are biased.
Instead of outlining the plot, I will just summarize that visually, this movie is more attractive to the eye than many of Elvis' other movies (that have too many studio sets instead of using outdoor shots). Although most of this film was shot in California than Florida, they pulled it off.
Elvis' movies, for the most part were "make you feel good" movies, so to judge this as anything more than that would be doing Elvis a disservice. This film did mark the beginning of the end for Elvis in a number of ways. It was the last film for which he received a guaranteed million dollar salary. He also began overeating prior to this movie and had to take diet pills to reverse the direction his weight was heading. No more needs to be said.
If you are like me and either have only a few Elvis movie DVDs in your collection (I have about 10 of his 31) and are wondering which ones are more enjoyable than others, I can only give you a general guideline and that is that most of his better movies were from the FIRST third of the 31. I don't think you can go wrong at all by picking any of his first five (Love Me Tender, Loving You, Jailhouse Rock, King Creole, and GI Blues) as well as Viva Las Vegas, Blue Hawaii, It Happened At The World's Fair, Roustabout, and Follow That Dream. I think these movies do a better job of showcasing Elvis' untapped acting potential. I wrote "untapped" because you could see if he was given better scripts (especially after his first five movies), he could have shown so much more of what he had to offer. Thanks a lot Hal Wallis!
Overall, I don't recommend this movie for purchase unless you have to have every movie in which Elvis has appeared.