Hydrocharitaceae is in Alismatales. Its flowers are usually dioecious with 3 free sepals and 3 petals (sometimes absent) . The ovary is inferior.
Hydrocharis (UK: Frogbit)
Leaves with long petioles and cordate blades. Roots hanging in water.
Stratiotes (UK: Water-soldier)
Sessile leaves, leaves in a basal rosette, leaves sharply serrate. Conspicuous white petals.
Egeria (UK: Large-flowered Waterweed)
Leaves borne along the stems, whorled leaves, petals white and bigger than 5mm
Elodea
Leaves in whorls of 3-4, rooted in mud. (Only female plants occur in Britain)
Hydrilla
Lagerosiphon
Sessile leaves, leaves borne along the stems, leaves not strictly arranged; variously whorled to spiral
plantfamilies 