something that looks difficult but actually isn't
Idiomatic expression likening a seemingly tough task to a kappa's mid-winter swimming practice, which is easy for the water-dwelling creature. Used to downplay apparent difficulty.
Think of your first presentation as something that looks hard but isn't, and take it easy.
彼にとって徹夜作業は河童の寒稽古のようなものだ。
For him, working through the night is like swimming in a cold lake for a kappa—seems tough but it's nothing.
From the folkloric creature kappa (河童), a water imp, and 寒稽古 (mid-winter training). The phrase plays on the idea that swimming in cold water is easy for a kappa, so the task only appears difficult to outsiders.