We see that the phrase "We all float (down here)" several times throughout the novel, coming either from Pennywise or from the dead kids.
The first time we come across this phrase is when George asks Pennywise if the balloon floats to which he responds with something along the lines of "The balloon floats and you will too."
Does It just end up liking the phrase and use it over and over again or does it have some other meaning?
In the book, Stephen King uses the word "float" nearly 120 times (117, actually). The word "float" as Stephen King used it in the book can be interpreted three ways...
Stephen King uses the word "float" several times to say, that an object (like a balloon or a log) literally floats. Some quotes where this is seen are as follows:
“Sure,” Ben said. He looked briefly at Beverly, who was standing close to Bill, and felt a pain he had almost forgotten. A new memory trembled, almost within his grasp, then floated away. It, In the Watches of the Night, 1
and
The Penobscot and the Kenduskeag were full of floating logs from ice-out in April to ice-in in November. It, Derry: The Fourth Interlude
and
The color was washing out of the world, and when Henry let go of him and pushed, he seemed to float toward the sidewalk... It, Eddie's Bad Break, 3
Many times throughout the novel as well Stephen King uses "float" to directly allude to It's presence or involvement. A few quotes where this allusion is made are:
The Standpipe was now on his [Stan's] right, a chalky white cylinder, phantomlike in the mist and the growing darkness. It seemed almost to . . . to float. That was an odd thought. He supposed it must have come from his own head—where else could a thought come from?—but it somehow did not seem like his own thought at all. It, Cleaning Up, 10
and
Henry’s voice, fury masquerading as mockery, floated down: “We can wait up here all day, you guys [the Loser's club]!” It, In the Watches of the Night, 12
and
“It didn’t hover,” he said. “It floated. It floated. There were big bunches of balloons tied to each wing, and it floated.” It, Derry: The Second Interlude
The final way in which "float" is used in the novel can be interpreted from It's intentions when he says "we all float down here". Although "float" as It uses it could be interpreted as literally floating down in the sewer (or hovering as the movie depicts it), the meaning which Stephen King is trying to convey to the reader is probably a little more than that.
To "float" as Pennywise intends it is to float in the Deadlights. The Deadlights in the novel is a place deep in the macroverse - where It originally came from - filled with orange light. Stephen King portrays to the reader that It has two forms:
You could almost say that It floats between the two, and that's what he essentially intends of his victims; their bodies may be destroyed, yet It has dominion over their minds in the Deadlights - his victims float in the median between physical and metaphysical death, never truly free.
You can kind of see this concept at play in the movie, when It separates Beverly from the others and shows her the spinning orange lights in his mouth - the Deadlights - to which she is temporarily lost.
Some quotes from the book where this concept is touched on can be seen below:
Somehow the [physical It] and the It which It called the deadlights were linked. Whatever lived out here in the black might be invulnerable when It was here and nowhere else . . . but It was also on earth, under Derry, in a form that was physical. It, The Ritual of Chüd, 2
and
...wait until you break through to where I am! wait for that! wait for the deadlights! you’ll look and you’ll go mad . . . but you’ll live . . . and live . . . and live . . . inside them . . . inside Me . . . It, The Ritual of Chüd, 3
and
The writer’s woman was now with It, alive yet not alive[...] Now the mind of the writer’s wife was with It, in It, beyond the end of the macroverse[...] She was in Its eye; she was in Its mind.[...] She swam in the deadlights. It, Under the City, 3
