Crucifer?

Crucifer?

Observation - Crucifer? - UK and Ireland. Description: Plant over 1m tall growing in water, so unable to get close. White four-petal flowers and long strap-like leaves. Any suggestions appreciated. I will probably kick myself when someone comes up with an ID. It feels familiar, but I can not put my finger on

Plant over 1m tall growing in water, so unable to get close. White four-petal flowers and long strap-like leaves. Any suggestions appreciated. I will probably kick myself when someone comes up with an ID. It feels familiar, but I can not put my finger on what it is.