SK 3183 29 November 2021: 7 very high res. images.
I tried all sp. from efn but unable to find a match.
And the stem underwater looks hollow !
Persicaria lapathifolia I hope
I guess Persicaria lapathifolia (L.) Gray is correct ID !
Looks very close to P. lapathifolia. However, P. lapathifolia will usually have large dark black/purple blotch on the adaxial part of the leaves. Therefore, I am skeptical.
Floral details and achenes should also be observed before confirming the species ID.
Most Polygonum species will have knotty stems, therefore, they are also called knotweeds. Other characters mentioned by you are common to many of its species. Therefore, it is difficult for me to confirm the species ID.
OK … The location being a national Park I could not collect the specimen !