What is the humor in "water's warm and deep" while relieving oneself?

The two men halted at the edge of the canal, hiked their tunics, then
fumbled with their loincloths. Soon two arcs were gurgling across the
filmy surface. “Hmm,” Xinemus said. “The water’s warm.” Even
terrified, Esmenet rolled her eyes, smiled. “And deep,” the Prince
replied. Xinemus cackled in a manner at once wicked and endearing.
After securing himself, he slapped the other man on the back. “I’m
going to use that,” he said merrily, “the next time I piss back here
with Akka. If I know him, he’ll damn near fall in.” “You’ll have a
rope to throw him at least,” the taller man replied. More laughter, at
once hale and warm.

— The Darkness That Comes Before by R. Scott Bakker

The water’s warm is probably referring to the warm sensation of pee but how does it relate to And deep?