Ceratophyllaceae . . . . Hornworts
The water plants are not particularly common and can be confused with the Algae, Chara and Nitella, which grow in similar places.
They can be separated by the degree of branching: (these 3 photos from C Hutchinson)
The two species
c.2x in C. demersum
3-4x in C. submersum
Ceratophyllum demersum . . . . Rigid Hornwort
Scattered in ponds and ditches in En and Wa; rare elsewhere
Ceratophyllum submersum . . . . Soft Hornwort
Less common than the above, local mainly in C & SE En; can be confused with the Algae, Chara and Nitella, which grow in similar places.