In The The Sea Nymph's Lair — Robert E. Vardeman

The ferocious Spell Wars have ravaged the island kingdoms of the Plenn Archipelago for more than a century, leaving behind ghostly remnants only wizards of Durril's ability can exorcise. With his apprentice, Arpad-Zen, Durril is at sea when an horrific storm sinks their ship. Arpad-Zen finds himself adrift--but Durril is rescued by magical denizens of the sea. But is being dragged to the bottom of the ocean and held in thrall by the sea nymph Rusalka a "rescue?" He is forced into Rusalka's service but she is never satisfied and wants ever more from him. First he is to perform an undersea quest that no one has ever survived and then there will be an even greater challenge...his nuptials with the ruler of the under-ocean kingdom.