Jess writes his composition on football because "he (has) enough brains to know that if he said drawing (is what he likes to do), everyone would laugh at him". Leslie writes about scuba diving.
The assignment had been for each student to write about his or her hobby. Jess hates football, but chooses to write about it because it is what boys his age are expected to like to do; "most of the boys (swear) that watching the Washington Redskins on TV (is) their favorite hobby". What Jess really likes to do is draw, but he knows that this is not an activity that will be looked upon kindly by his peers. Jess does not have enough confidence to expose himself to the scorn and censure of his peers. Lacking the courage to be true to himself, he takes the easy way out and goes along with the crowd.
In contrast, Leslie writes about one of her true passions, which is scuba diving, and does so with such skill that Jess is "all in a tremble just listening to Mrs. Myers read about it". Jess is amazed that Leslie has the courage to participate in such a different sport, "that she (isn't) scared of going deep, deep down in a world of no air and little light". Not only is she unafraid to engage in scuba diving, but she is also fearless enough to tell about it, even though none of her classmates have ever done anything like it. Leslie, unlike Jess, has enough confidence just to be herself, even if she doesn't fit in with the rest of the kids in the class.
Leslie's composition makes Jess like a coward, both literally and metaphorically, in comparison to her. He would be scared to go scuba diving, to explore the world "under the dark water", and he is also scared to let people know whom he really is, to let them know that he likes drawing more than football, just as Leslie likes scuba diving in lieu of more socially acceptable activities like "watching game shows on TV, and...reading Good Books" (Chapter 4).