inundo

The Latin seam is real. The Esperanto seam is not. Inundare genuinely is in- plus unda, "to wave into", and if you know that, the word stops being arbitrary. But do not carry that analysis into Esperanto, because neither piece is an Esperanto piece: the prefix for into is en-, not in-, and the word for a wave is ondo, not undo. Split inundo the Esperanto way and you get two things that do not exist. This is the same class as inspiri: a borrowed word whose internal history is genuine and instructive, and whose seam is invisible to the language that borrowed it.

Which makes ondo worth a second look. Esperanto's wave is ondo, from the same Latin unda by way of French onde, which lost the n-before-d nasal that Latin had. So ondo and inundo are cousins that no longer look alike, and Esperanto took them in from different places and different centuries. Ondi is to wave or undulate, ondego is a big wave or a breaker, and radioondo is a radio wave. None of them will help you spell inundo, but they will stop the word feeling foreign.

Naming the event. Inundo is the flood itself, the water where it should not be. Inundi is transitive: la rivero inundis la kampojn, the river flooded the fields. Superakvego and akvoinundo turn up in older texts and are unnecessary. For the figurative sense, Esperanto uses the verb freely, as English does: inundo de leteroj, a flood of letters, la merkato estas inundita, the market is flooded. And a flash flood, which arrives without warning, is subita inundo.

Around it. Superflui is to overflow, which is what a river does before there is a flood. Diluvo is the Deluge, the biblical Flood, and it is not interchangeable: inundo is a weather event, diluvo is an act of God or a joke about one. Katastrofo covers the aftermath, and inundata regiono is a flood-prone area, using the passive participle to say "regularly being flooded" rather than "flooded once". The participles article explains why the tense of the participle is doing that work.