trinki

Trinki is transitive and takes a plain accusative, so trinki akvon with no preposition; the source of the liquid takes el, as in trinki el la rivero. The affixes do the rest of the work you would expect: trinkaĵo is a drink as a thing, trinkejo is the place that sells them, and trinkema is a polite way of calling somebody fond of a glass. Note that Esperanto keeps eating and drinking strictly apart - unlike English soup, which you eat, supo is normally manĝata, so the pairing is not a safe guide.