'Arsonist' is to 'fire' as what is to 'water'?

The word arson derives from the Latin word for burning; the Latin word for flooding is inundo. So your closest match is something like inundationist, though I don't think this word is attested, and inundate isn't considered criminal.

Swamper is a possibility, in its sense 3, “Someone or something that swamps or overwhelms”. Also consider flooder, a word of more recent origin meaning “A person who floods message boards with unwanted or repetitive comments”. Drencher means “One who, or that which, drenches”. Deluger would have similar meaning but isn't in most dictionaries. Related: dambusters, those who bust dams, causing floods.