Reed-mace (Typha latifolia)
In fact, if you look in a field guide of grasses, sedges, rushes and reeds you will not find this plant at all, you need to look in a wild flower guide as, although it thrives in similar habitat to reeds and sedges it is totally unrelated.
I will leave it to real botanists to muse over why this is a flower and not a grass but, regardless of its classification, is a 'functional' plant. The attractive brown pods it produces are packed full of seeds which split when ripe and the seeds fall, or are blown, on to the water where they get gradually get washed to a muddy area where they settle, germinate and produce more Reed-mace.
Not a great food source for insects perhaps but Reed Buntings and other birds do like the seeds.