Submarginal, so usually found on the riverbank but will venture into the water of its shallow enough, and can be distinguished by its lateral growth, slightly hairy ovate leaves growing in whorls around the main stem, purple to pink flowers and of course like it's cultivated cousin it's unmistakeable fragrance! If in doubt snap a bit off, crush the leaves and have a sniff. If you've got the minty fragrance you've got it, if not keep looking as it probably won't be far away!
Uses are that of cultivated mint, tea, sorbets, sauce to go with your lamb and of course, a wild mojito. Richard Mabey, author of the legendary Food for Free has a great recipe for Water Mint Indian style chutney.
The flavour differs from cultivated mint with sweeter, deeper flavour. Care needs to be taken to not harvest from water sources near to livestock to avoid the parasitic liver fluke, known to inhabit plants near water sources and looking for a mammal to continue their life cycle. Whilst cooking/boiling the leaves removes the microscopic threat we recommend just leaving anything growing near to farms or livestock.