The sloop tossed about on the stormy sea, and its starving passengers suffered from great privation. Yet the had no conception of how much worse the situation could get.
A sloop is a sailing vessel that usually has one mast. Historically a sloop was a two or three masted ship with square sails. Privations are the loss of things that are considered essential for living. The people are suffering from a lack of food and water. This is shown through the description of the passengers as starving. Conception is similar to concept or idea.
Coopering is the repairing of a barrel or cask. It is not used in this passage.