Calling the romantic subplot a "waste of time" is being exceedingly kind. It doesn't just not belong there, it should be charged with some kind of crime. During the scenes between Willie and his girl (and his mom) I wanted to either kill them or myself or both, they are beyond excruciating. I'm really not sure if it was in the book, but I hope not. It reads like something that was tacked on to appeal to women, but only the mentally defective variety.
However, in the book it was a fully formed storyline and far deeper characterization.
The book, essentially, is about how Willie Keith grows up and becomes a man. The two things that cause him to do this are: his relationship with May and his service on the Caine. (In the book, he's never sent to another ship after the court-martial. He returns to the Caine, spends the rest of the war aboard her, saves the ship during a kamikaze attack at Okinawa and is named her final captain before it's decommissioned and sent to the boneyard.)
It follows Willie from his meeting with May in late 1941, through his service on the Caine and his return from the war in the autumn of 1945. May is a major character in the book. She's doubtless the most important non-naval character who spurs Willie's development.
Willie's father is in the book's early chapters. He dies of a melanoma soon after Willie leaves for the Pacific. At one point he visits Willie at midshipman's school. Willie -who's distracted by his relationship with May- tells his father all about her. (He'd not told his family about her yet.) His father actually approves of her. He says that in his opinion, both May and the Navy are having a positive influence on Willie and he'd like to meet her someday.
I agree in the film it was really a needless subplot. But in the book, it was one of the two main narratives of the story.
Well at least the "Willie's love life" scenes are kept to a minimum. I had no problem with them because the whole story is seen through Willie's eyes. His personal life is part of the story and it has some effect on his perceptions and growth in his Naval life.
Thankfully, the romantic angle, didn't consume most of the film. I think you're right though, I do believe it was tacked on, to appeal to women. I don't mind romance in films, as long as it's in the proper context, and this was not one of those films.
It's really insulting to the intelligence of women that movie producers assume they won't be interested in an engrossing story about human interactions if those humans are men doing serious jobs, and that a boy-girl romance has to be tacked on to get their attention.
It certainly didn't hold up with time! At the time the decision to include it in the movie was probably to give the "gals" a reason to go watch the movie with the "Guys".
A Swiss Army knife does a lot of things. But it can't do any of them very well.