In Dune, Are the Guild Navigators non-humanoid?

The other answers covered the original Frank Herbert books.

But the prequels (by Brian Herbert and Kevin J. Anderson) expand that information significantly and very explicitly show that Navigators are simply spice-mutated humans.

• Origin

  The very first Navigator was Norma Cenva, and the details are covered in The Battle of Corrin:

  Her direct physical senses were deadened, and Norma no longer cared about taste, touch, or smell ... She found it remarkable to see webbing between her fingers and toes. Her face, once blunt-featured and later flawlessly beautiful, now had a small mouth and tiny eyes surrounded by smooth folds. Her head was immense, while the rest of her body atrophied to a useless appendage

• Dune times

  More contemporary storyline close to Paul Atreides' time was the story of how D'murr Pilru became a Navigator (he was a human from Ix) in Prelude to Dune trilogy.

I don't have a ton of quotes handy, but as you probably recall towards the end of the first book, the guild navigators have the blue eyes indicating an addiction to spice. As others have indicated, Dune Messiah (second book) is really the first description of a steersman (Edric) who is described as a fish-like being who constantly hovers in a tank filled with spice gas.

In my mind, I always pictured navigators being smaller than the image above. I am currently reading the second book, so I may (probably am) wrong in my guess, but I assume that a navigator's fish-like body is some mutation/side-effect of super-serious spice addiction, since the navigator essentially is 24/7 suffocated with spice.

In the first book (Dune), the only description we have is of two of their agents, who're described as "men" with no indication of non-humanoid features:

He and his companion pushed through to the barrier lances, which were raised at a nod from Paul. The two men stepped out and the taller leveled an arm at Paul, said: “You may very well be under embargo for your—”
“If I hear any more nonsense from either of you,” Paul said, “I’ll give the order that’ll destroy all spice production on Arrakis … forever.”
“Are you mad?” the tall Guildsman demanded. He fell back half a step.
“You grant that I have the power to do this thing, then?” Paul asked.
The Guildsman seemed to stare into space for a moment, then: “Yes, you could do it, but you must not.”
“Ah-h-h,” Paul said and nodded to himself. “Guild navigators, both of you, eh?”
“Yes!”

It's not until the second book (Dune Messiah) that finally we meet a true Steersman, one of those capable of piloting a heighliner;

Scytale shifted his gaze from Irulan to the tank, inviting the Princess to share his viewpoint. She would, Scytale knew, see Edric as a repellent figure: the bold stare, those monstrous feet and hands moving softly in the gas, the smoky swirling of orange eddies around him. She would wonder about his sex habits, thinking how odd it would be to mate with such a one. Even the field-force generator which recreated for Edric the weightlessness of space would set him apart from her now.

and

“You know what is said of Alia?” Scytale asked, probing.
“What do you mean?” Again, the fish-man was agitated.

The appearance from the movie (assuming you are talking about the David Lynch version; I haven't seen the others) is not inconsistent with the books.

As far as I can tell, it's only alluded to in Dune (Paul mentions his intent to spot a Navigator, only to be scolded by Leto); but in Dune Messiah, it's made quite clear.

In Dune:

“I’m going to watch our screens and try to see a Guildsman.”

“You won’t. Not even their agents ever see a Guildsman. The Guild’s as jealous of its privacy as it is of its monopoly. Don’t do anything to endanger our shipping privileges, Paul.”

Then, in Dune Messiah, right at the beginning: