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.
