I'm reading Lawrence Block's memoir, in which he mentions about volunteer travel opportunities offered by Earthwatch. He explains that for a not-too-steep fee, one might join an expedition in aid of some environmental goal, such as to save the endangered red pandas. Then he says:
Lynne and I weren't sufficiently high-minded to sign up for one of their trips. We’d be eager to inoculate villagers in the Cameroun against dengue fever instead of, say, knocking back on a cruise of the South Pacific.
What does "knocking back on a cruise" mean here? I tried to look up the dictionary for "knock", "knock back", or "knock back on". But their definitions don't seem to fit this context.