This is a summation of episode 5 of the Landscape season 3.
The adventurers head out of Esakar Landing to find Sir Saallizaar, Knight of the Bronze, expert anthropologist and Sahuagin observer. They find him surfing outside of his ramshackle bamboo shack and enlist his help to investigate the explosion in Sahuagin raids along the coast of the Razor Sea.
Saal takes the group out onto the ocean and above a deep and magical underwater trench that he knows is home to many Sahaugin tribes. Together they dive into the trench, and steathily swim into the dark depths.
Once in the trench, they encounter a pair of Sahaugin submersibles trawling the trench walls. They attempt to dispatch the pilots quickly and silently, but one manages to raise a signal - a soluble blood bomb launched from the belly of the submersible - that attracts the attention of a huge dire shark.
The experienced and powerful adventurers make quick work of their foes, and investigate the submersibles to find a series of charts and maps - incredibly valuable and rare - that lay out the hazards and settlements within the trench!