"The 3 humans wander through forest and scrub, through 'pockets of temporal distortion' and a sort of lake..."

"... so subtly underwater the newlyweds only notice they are submerged when the gun won’t fire."

They hunt (“That was rather exhilarating!” says the husband, knocked over by his own recoil), and are hunted in turn—menaced by a mobile, anthropomorphic forest grove, swallowed by a furry, almond-shaped monster with a very yonic mouth, chewed on by a four-legged, bodiless cyclops. Parasites threaten their orifices—the guide pulls a hissing worm out of the husband’s ear, but is later infected himself—and ethereal, telepathic “forest apes” watch them from afar. At one point Jacobs suddenly pulls back to show that the whole forest is actually the face of a maniacally grinning giant, a “revelation” that is never returned to.

The book is "Safari Honeymoon" by Jesse Jacobs.