The hydronium ion is our conception of the acid principle of water. Another conception is #H^+# (and please remind yourself what a conception is!). So what is the acidic species in water? It is a cluster of water molecules, 3, or 4, or 5, with an extra proton, #H^+#, associated with the water cluster to give (say) #H_7O_3^+#, or #H_9O_4^+#. In pure water, what is the counterion? And of course this cluster rapidly exchanges the #H^+# with other clusters. It is simpler to treat this cluster, this species, whatever is it, as #H^+# or #H_3O^+#, protium or hydronium ions.
