I looked up that part. Indeed there are two of the "guild agents, one tall and fat, one short and fat" as part of the entourage, and they are referred to in the text as Guildsmen. Paul asks them if they are both navigators and they reply with yes. And yeah they are revealed to have blue eyes, so I think this is correct. However, later books do describe the tanks in which the navigators live.

The dune wiki says that Herbert apparently liked how the navigators were depicted in the movie so he added their description to his last Dune book "Chapterhouse Dune". But even earlier books described the guild navigators as humanoid fish in a tank.

Apparently the explanation for these different forms is that there is a progression, where the guild navigators start out looking like humans, and eventually they look like the two in the Lynch movie.

https://dune.fandom.com/wiki/Guild_Navigator#Physical_Descri...

So what I said about the navigators not appearing at all was wrong, as they do appear in the first book, but they appear only way later in the story.

There’s mention of “stages” of navigators in one (or more?) of the books. Stage 3, as I recall are the advanced form in the tank.

Actually, that wasn't an invention of David Lynch. It was just something sort of pulled in from the second book.