They mostly suck. Theodore Sturgeon coined the principle- "Sturgeon's Law"- that says "90% of anything is crap." With novelizations, it's closer to 99.9%.
You're probably okay to dodge this literary genre.
Now and then you can learn a thing or two from them. The Gremlins novelization reveals Gizmo's alien origins. The Aliens novelization has Newt's origin story. The Dark Knight Rises novelization reveals where Joker was during the events of that film. The original Shaun Hutson Terminator novelization reveals that the Terminator had feelings about what he was doing. (Which was not canon and one of the big reasons James Cameron blocked its publication in the U.S.)
But the most important thing you can learn from novelizations is what not to do.
With the exception of the superbly crafted Orson Scott Card adaptation of The Abyss- arguably even better than the film- novelizations are a debased art form. They are by necessity rushed and sloppy, since the author is given an early draft of a script to work with and is then forced to crank it out in a few days or weeks to make sure it meets the publication deadline.
If you've ever glanced at the spines of a few novelizations, you probably noticed that almost all of them are written by Max Allan Collins (Though Alan Dean Foster wrote all the ones that Collins missed). And the Waterworld novelization is no exception:
But the thing about this one is...it's oddly good.
It's disarming from the start, with an extremely well-written prologue and solid character development and narrative flow.
He has a great eye for detail on the Mariner's ship right off the bat:
"Wind chimes fashioned from ancient computer boards and printed circuits tinkled and sang a melancholy tuneless tune; a prow-mounted harmonica played its own ghostly nonmelody; and cockpit controls shifted idly with the current."
Everything about that description is great. Effortlessly shows how the highest tech of the past has become the tinkling trifle of the present. And feels trenchant with a longing for the shattered past.
Its main limitation is the source material. Waterworld- when you get past its legendary status for the amount of money it burned through- is still just a generic post-apocalyptic Road Warrior rip-off with water in place of sand. I don't hate it- it's entertaining pulp- but it's not a story for the ages.
And this is about as good as a novelization can possibly be. I have no idea why Max Allan Collins put so much care and effort into this.
My friend got this for me as a joke. We have a running gag where we seek to find the worst possible books and give them to each other on birthdays and Christmas. (e.g. Mortal Kombat: Annihilation novelization, Daredevil novelization, bodice-ripper romance books, Vanilla Ice's autobiography, etc.)
But the joke's on him, because this one wasn't that bad. In yo face!