They are surprisingly maneuverable, though:
In typical engineering, stability and maneuverability are often opposing goals. Boxfish have evolved to embrace instability — but with remarkable control. Their body shape increases drag slightly compared to more streamlined fish, yet that compromise enables agile turns and precise swimming within coral labyrinths — vital for escaping predators and navigating terrain.
Even if it was just the adults, them not doing it eliminates herd immunity and puts others at risk.