Some similarities to :
Alpine Water-moss (Fontinalis squamosa) but that does not have flat leaves as does Greater Water-moss, but rounded leaves only half as long and when closed takes on a cylindrical overall shape rather than a triangular prism.
Slight resemblance to :
Filmy-Ferns such as
Wilson's Filmy-Fern (Hymenophyllum wilsonii) and to
Holly-leaved Naiad (Najas marina).
No relation to : Water Fern (Azolla filiculoides) [a plant with similar name].
Greater Water-moss grows in lakes, ponds, streams, canals and rivers that have still, slow-flowing or rapidly moving waters. It prefers nutrient-rich and neutral or less acidic waters (
Alpine Water-moss prefers more acidic and nutrient-poor waters). It tolerates being out of the water at times, closing up to conserve water. A canal-lock may be ideal for it with the constant churning of water, aeriation of it, and the intermittent flow of fresh nutrients. That being so, it is strange that it does not inhabit most locks, but perhaps that reflects the poor water-quality in many of the canals?
It is widespread throughout the UK with only few hectads where it is not found.