We get the best comparison of these two women aboard the ship on the Atlantic, when together with Charles they form a neat little love triangle. Since Charles is married to but resents Celia and in love with Julia, the stage is set for nice little foil.
First, look at the dinner conversation when they’re all seated at the Captain’s table. Celia is thrilled to be a VIP; Julia gives Charles "a little dismal symbol of sympathy" – an emotion much more in line with Charles’s own opinion of the seating arrangements. While he and Julia banter cleverly about King Lear, Celia is left in the dark. "Don’t [even] try to explain," she insists to them. If Julia is tom-boy masculine, Celia is delicate-flower feminine, with her "softness and English reticence, her very white, small, regular teeth, her neat rosy finger-nails […] her peculiar charm." Notice that Julia is the only woman up and about during the storm with the men, while Celia lies in bed sick from the tossing seas. Everything that attracts Charles to Julia similarly explains why he could never be happy with a woman like Celia.