Pearls and Scimitars (The Jinni and the Isekai, #6‪)‬

Cut off by the Yamu, Shiro and his friends are forced to go farther down the Eiphr River than they had originally intended. Still, they lead their Scorpions toward Southern Kalush where they hope to flank the Florencian advance upon the satrapy. But first the adventuring army must cross the straights, known anecdotally as the Isles of Sand and Bones—a crossing in the tributaries at the mouth of the sea consisting of elongated islands and lagoons not explored by any Abassir man for centuries—and for good reason. The straights are said to be a region of primal magic and monsters so dangerous that even the most powerful adventurers have avoided them.

Once the Florencians are properly dealt with, perhaps Shiro and Jessamine can finally adventure to discover the mystery behind his isekai nature—if they manage to survive, that is...