Description from Flora of China
Herbs, perennial or annual, in fresh or brackish water, totally submerged or with floating leaves. Stems terete to compressed, rarely strongly compressed. Leaves alternate, occasionally opposite, mono- or dimorphic; stipules membranous, free or partially adnate to leaf base (sheaths); submerged leaves sessile or petiolate, linear or with thin blades lanceolate; margins entire, denticulate, or serrulate; floating leaves petiolate, with leathery blades lanceolate to ovate or broadly elliptic-oblong. Inflorescence a pedunculate spike, terminal or axillary, surrounded by sheath in bud, emergent, floating or submerged at anthesis. Perianth 4-merous, free, bractlike and shortly clawed, each inserted opposite a stamen. Stamens 4, united with perianth at base; anthers sessile, 2-celled, extrorse, dehiscence longitudinal. Carpels (1-)4(or 5), free; stigmas sessile or on short styles, expanded, capitate or peltate; ovule solitary, attached to adaxial side of carpel. Fruit drupaceous with fleshy exocarp and bony endocarp. Embryo curved or spiral, rarely erect; endosperm absent.
Hybridization has been recognized as frequent even in the genus Potamogeton. This situation not only obscures the limitation of some related species but also makes difficulties in the treatment of many infraspecific units in the genus. Exceptionally with the confirmed 20 species, there are ca. ten or even more speculated hybrids in China.
About 75 species: cosmopolitan; 20 species in China.