Whirlpool lagoon summary
The ‘lagoon’ that the whirlpool is shaped kind of like a kidney with the whirlpool in the SE lobe… above the whirlpool itself, two pipes about 2 ft in diameter spill filth from about 10 feet above the water into the lagoon. The ‘water’ of the lagoon spins up to 30 feet or so away from the core of the whirlpool itself.
Other than the whirlpool, the ‘river’ per se flows from the north (where the dragon is chained to the big frog demon altar) and then out to the West where it flows lazily past the shattered wall landing area in the medusa room, which is where you guys got the boat from.
Recall that the angelic porpoise that Xaen summoned almost immediately foundered as it approached the whirlpool and was sucked down, never to be heard from again… so clearly the power of the suction of the whirlpool is pretty strong.