[...] Thus, it bonds to the oxygen atom of a solvent water molecule to give trigonal pyramidal hydronium ion, $\ce{H3O^+}$ $\ce{\{[H (H2O)]^+\}}$ (see box). In this chapter we shall use $\ce{H^+(aq)}$ and $\ce{H3O+(aq)}$ interchangeably to mean the same i.e., a hydrated proton.
Isn’t this the way to write coordinate compounds? This may sound dumb but is the hydronium ion a complex compound?
The box that this passage refers to:
Hydronium and Hydroxyl Ions
Hydrogen ion by itself is a bare proton with very small size (~$\pu{{e-15} m}$ radius) and intense electric field, binds itself with the water molecule at one of the two available lone pairs on it giving $\ce{H3O+}$, This species has been detected in many compounds (e.g., $\ce{H3O+Cl-}$) in the solid state. In aqueous solution the hydronium ion is further hydrated to give species like $\ce{H5O2+}$, $\ce{H7O3+}$, $\ce{H9O4+}$. Similarly the hydroxyl ion is hydrated to give several ionic species like $\ce{H3O2-}$, $\ce{H5O3-}$, $\ce{H7O4-}$, etc.
Source Chemistry Part 1, Textbook for Class XI National Council of Educational Research and Training (https://ncert.nic.in), NCERT Publications (version: October 2019 Ashwina 1941) https://ncert.nic.in/textbook/pdf/kech107.pdf