I'm not sure I can recommend anything beyond my personal preferences, which generally tend to run to the humorous for my pleasure reading, though as a kid I devoured adventure books.
If your husband like contemporary comedic fiction, I'd recommend anything by Christopher Moore--the closest diving-related book of his is
Fluke: Or I Know Why the Winged Whale Sings. I have a friend who swears by Chuck Pahlaniuk, though I've yet to start reading his books. Of course, there's also Kurt Vonnegut, who just passed away last year.
If your husband enjoys well-told tales of high-adventure that have stood the test of time, I'd recommend
Scaramouche,
Treasure Island,
The Three Musketeers, or
The Count of Montecristo (the last happens to be my dad's favorite book--a belated Father's Day shoutout to my dad!).
If your husband is a bit of a sci-fi/fantasy nerd, I'd recommend anything in Terry Pratchett's Discworld series, Douglas Adams's Hitchhiker's Guide to the Galaxy series, and the early books from Robert Asprin's Myth series. For the most part, the books in the Discworld series are all stand-alone novels, so your husband can pick any one of them up and begin the series there. The Hitchhiker's series and the Myth series generally are better reads if read in order. The first book in the Hitchhiker's series, surprisingly, is titled "The Hitchhiker's Guide to the Galaxy"; the first book in the Myth series is "Another Fine-Myth".
If your husband likes comedic mystery, I'd recommend any of the earlier books by Carl Hiaasen, Tim Dorsey, or even Dave Barry. I think there's probably something in the Florida waters, since these guys all write about some really "unique" situations. Oddly enough, I think Carl Hiaasen has been quoted as saying that many of the stranger story elements that he's put to pen actually occurred in real life (for instance, a male dolphin that got a little to "friendly" with a female swimmer in the dolphin tank).
If your husband likes being able to put down a book (for instance, once a surface interval is complete) without having to leave a book at "the good part" but still wants to be entertained by good storytelling, I'd recommend a book of shortstories by Mark Twain, O. Henry, or Damon Runyon. While Twain is well-noted for his humor, there's something captivating in the way he spins a yarn regardless of whether it's comedic or serious. As for O. Henry, his stories tend to have an unexpected twist at the end (though I must admit that after a while, I tend to look for the unexpected twist). Damon Runyon's tales tend to revolve around memorable and sympathetic characters, though not all of them are of the "cute-and-cuddly" kind.
If your husband enjoys comedic plays, I'd recommend Plautus (classical Roman playright), Moliere (a French Baroque playwright), or Neil Simon (mid/late-20th century American playwright). One of my favorite Plautus plays is "The Golden Ass". As for Moliere's "The Misanthrope" I find it a little dated but still funny. "Rumors" by Neil Simon is hilarious--I was first introduced to it by my high school drama department.
If all else fails, you could always get your husband a portable DVD player, some batteries, and a collection of DVDs to last the entire trip!