The underground river shouldn't be the first thing the Party hits- there should be largeish dry areas with ponds secretly attached by hidden tunnels...
Eventually, the passages would tie into an underground river...possibly a lake or sea...
Which, of course, leads to grabbing stuff from Journey to the Center of the Earth or any of the various Hollow Earth mythologies.
As I recall, there are some caves that have a mix of miles and miles of tunnels interspersed with isolated pools that are actually exposed to the surface & sunlight. Some enterprising predators might even use that like a polar bear uses a hole in the ice...
Piercers, bats, spiders & other arthropods, blind cave (fill in the blanks)...don't forget that Undead are always an option, especially if they are either dummy training targets for the Sahaugin commandoes or shock troops for the raiders, under the command of a Sahaugin Necromancer.