The optical properties of the bulk ZrSiS nodal-line semimetal are theoretically studied within a many-body formalism. The G0W0 bands are similar to those calculated within the density functional theory, except near the Γ-point; in particular, no significant differences are found around the Fermi energy. On the other hand, the solution of the Bethe-Salpeter equation reveals significant excitonic activity, mostly as dark excitons which appear in a wide energy range. Bright excitons, in contrast, are less numerous, but their location and intensity depend greatly on the polarization of the incident electric field, as the absorption coefficient itself does. The binding energy of these excitons correlates well with their spatial distribution functions. In any case, good agreement with the available experimental data for absorption/reflection is achieved. Finally, the possible activation of plasma oscillations is investigated. Plasmons may be formed at low energies, but they are damped and decayed producing electron-hole pairs, more importantly for q along the Γ-M path.