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.