daf bit: Kiddushin 13

Rav Yehudah said in Rav's name: one who does not know the peculiar nature of divorce and betrothal (I think this means one who doesn't know the laws of how to effect them) should have no business with them. R' Assi said in R' Yochanan's name: such people are more harmful to the world than the generation of the flood, for it is written: by swearing, and lying, and killing, and stealing, and committing adultery, they spread forth, and blood touches blood (Hosea 4:2). And it continues (4:3): therefore shall the land mourn and everyone that dwells there shall languish with the beasts of the field and the fowls of heaven and even the fish of the sea. But with the generation of the flood nothing was decreed against the fish, because it says: all that was in the dry land died -- what was in dry land, but not what was in the sea. So those who commit Hosea's list of transgressions cause a punishment more harsh than the flood, and the rabbis understand that to include those who try to act on betrothals and divorces without knowledge. (13a)

Whatever Hosea is talking about involves a penalty more dire than the flood (it all comes down to the fish). How do we get from Hosea's list of transgressions to those who don't know what they're doing with divorce and betrothal? I think it's about the adultery -- if you bungle a divorce then she's still married but doesn't know it, leading to adultery. This is my speculation; it's not completely clear to me in the text that this is the reasoning.