This is Chapter 8, in Volume 2 of A Place Further Than the Universe
The ship sets sail for Antarctica. In short order, the girls are incapacitated with seasickness. The other expedition members seem so strong, but the girls can hardly move. As the voyage progresses, the waves get even more rough, rocking the ship tremendously. The girls face the fact that this is the journey that they chose. Going out on deck at night, they get wet from the splashing waves, and find their enthusiasm return. They feel better the next day, as the ship comes in sight of icebergs.
This chapter is based on Episode 8 of the anime.