The Torian Pearls by Jeffrey Lord

The narrative centers on Richard Blade, a unique human-computer hybrid who serves the British government by making dangerous, repeated trips into perilous alternate realities collectively known as Dimension X. In his latest journey, Blade finds himself in a planet plagued by rising seas, quickly joining the nomadic Kargoi people who are struggling to survive both natural disasters and organized attacks from monstrous wildlife. Blade soon discovers that these attacks are orchestrated by the highly advanced, nonhuman Menel, whom he confronts and neutralizes by destroying their technology, allowing the Kargoi to escape to a new land. Leading the Kargoi as their new chieftain, Blade establishes critical alliances with the native Hauri fishermen and the powerful kingdom of Tor to fend off an invading, gunpowder-equipped army known as the Vodi. After the overwhelming victory, Blade cements the new political coalition by marrying Queen Kayarna of Tor, thereby securing the future of the human races in this dimension ...