'Must' occurs twice more in quick succession shortly after that, between otherwise past tense verbs in the passage.Now instead of running before the storm they must row across the wind's way, and it was hard; waves striking the ship abeam pushed her always south of their new course...
Just prior to the above quote, the master of the ship has been speaking to the hero, Ged: But he said to Ged, 'Boy, you speak like a Seamaster, but I tell you...'.