I've believed this plant to be watercress ever since it was included (with warnings of its aggressive behavior) with the plant package by the company that installed our pond. I just noticed that the leaves are rather different from those in most pictures of watercress (see
Watercress (Nasturtium officinale)), and the flowers don't look quite the same either. The leaves do taste a bit peppery, so perhaps it's a relative? The photo below was taken just yesterday, giving an idea of flowering time.
