Apium nodiflorum

Helosciadium nodiflorum (formerly "Apium nodiflorum"), commonly called fool's-water-cress, is a flowering plant found in ditches or streams and native to western Europe. It is a low-growing or prostrate perennial with pinnate leaves which have a vague resemblance to those of watercress. "Helosciadium nodiflorum" has short-stalked umbels of very small white 5-petalled flowers which are opposite the leaves and grow from the side of the stem at the leaf axils. It blooms in July and August.
It is not a poisonous plant, but it could be easily confused with the allegedly poisonous lesser water parsnip – "Berula erecta" (Huds.) Coville. 
It is common throughout England, Wales and Ireland but is much less so in Scotland.
Hybrids.
There are a number of hybrids involving "Helosciadium nodiflorum":