One thing to notice is that adverbial descriptions are quite compressed. Dahl says his mother 'calmly handled the tiller'—he leaves you to imagine what that actually looked like.
Try to picture it for yourself. What does calmly handling the tiller look like to you? You might imagine his mother sitting up straight, but relaxed, looking across the water at their destination, not looking around at other waves, one hand draped on the tiller, making only the slightest adjustments as they drive across the water.
Why doesn't Roald Dahl describe his mother in more detail like this? He wrote more detail about the boat and the waves and getting drenched. Why compress his mother's action into a single adverb?
The answer might be to enhance the contrast between the wild scene and his mother's calm behaviour.
He spends a lot of time setting up the sense of wildness—the little boat, the mountainous waves, and drenched children—and then he hits us with a contrast: his mother calmly handling the tiller. That compressed description is part of the effect—it helps us feel her calm during an otherwise wild scene.