Trapa natans (Water chestnut) is a floating plant native to Europe and is hardy in its overwintering nut form, but not in its green floating form, as we are at the very northern limit of its natural spread. The nut develops attractive diamond shaped toothed floating leaves in late Spring and the nut acts as an anchor weight in shallow water. I find it quite hard to keep in good condition, I think it thrives on warmth, not our dull drizzly Devon days!
It’s generally available June to September, but is getting hard to come by so may only be available infrequently and in small numbers. Trapa bicornis may be substituted when I cannot source T natans