The description of a sea voyage is an apt metaphor to describe the experiences of the four children and of Mrs. Olinski, and it is one with which Mr. Singh is completely familiar. Mrs. Olinski, Noah, Nadia, Ethan, and Julian have all returned from journeys of self-discovery, and since their "travels" have all had a beginning and an end and involved experiences in uncharted "waters", their journeys can very well be compared with sea voyages. Noah had journeyed to Century Village, adjusting to life and learning to appreciate the older residents there. Nadia had also gone to Century Village and had discovered both how to adjust to her parents' divorce and how she could make a difference in the world by helping the turtles return to the Sargasso Sea. Ethan had taken a short but significant journey on the schoolbus, where he found that he had within himself the ability to overlook the detriment to his own social standing and extend himself to someone else in need, and Julian, after long years of moving from place to place, had found a place to belong at Sillington House, with the Souls. And finally, Mrs. Olinski had been on a journey to reclaim her old life, returning to teaching after having suffered a terrible accident.
Until he had come to Sillington House, Mr. Singh had made his living working as a cook on cruise ships with his wife, who was a singer. Their son Julian traveled with them until he was of school age, after which he attended boarding school and continued to travel with his parents during the summer. The metaphor of a sea voyage would come naturally to Mr. Singh because of his own family's experiences, and, in describing journeys, it makes complete sense that he compares them to traveling at sea.