Buck is scared of swimming. Skint absolutely took the approach of just throwing him into water and that was how Buck “learned,” so he associates water with being scared for his life. He is very capable of swimming, but will never choose to do so unless necessary. Jules, on the other hand, loves swimming. Growing up around the ocean and pools, she’s been in water her whole life. She will find any excuse to swim.
This doesn’t come up until there’s a youth group trip to Trickle Lake and Buck really tries to convince Jules to skip with him. They don’t like being with the religious kids anyway, right? But Jules knows this is one of her few chances to swim and begs him to come with her so she won’t be stuck having to do the “boring part” of youth group by herself. Buck reluctantly agrees, but when they get to the lake, he won’t even go near the water. Jules stubbornly refuses to go enjoy herself unless she knows what’s going on with him and she can be sure he’s okay. After he explains his fear, she brings him over to the dock and tells him to sit on the edge so they can still talk while she’s in the water. She has no idea, but Buck thinks about that interaction for years any time the topic of swimming comes up. He’ll never forget what it felt like to know that Jules cared enough to make him feel safe, without asking any questions or judging.