While there are some interesting plants to grow in wet places or shallow water when you're in the subtropics or tropics there is not so much choice in colder climate.
We have dug some ponds, but they only receive rainwater runoff and might be dry at some stages. I am searching for edible ideas. We get up to -5C
So far I have:
Oenanthe javanica (hope spelling is right, identification is important there are poisonous plants in this species)
Gingers (some varieties survive here)
Canna edulis (not really productive here)
Mints (thanks I've got enough)
Lebanese cress (grows like a weed)
Sagittaria (haven't harvested yet)
calamus (I don't know for sure which strain I have)
What else could I plant?
I've got a fair bit of wet area to contend with as well, I've been compiling a (hopefully) comprehensive list for our site in 6a. I've got it on my home computer and will try to remember to post it tonight.
Have you considered plants that are not eatable but useful? Coppiced willow or other species will give you material for baskets, wattle fences, bean poles, and more. Plants that produce great biomass in wet areas can be chopped and taken to infertile places as mulch.
The ultimate goal of farming is not the growing of crops, but the cultivation and perfection of human beings. - Masanobu Fukuoka
FORBIDDEN? I'd take that as a challenge, not an obstacle. Stick it to the man!
Elderberries are a good wetland species. You might also talk to your localnative plant society and see if they have a list of species. Then, take that list and find out what's edible via further searches.
I did not realize you are not in North America. Here there are companies that will ship wet seed. Whether they would ship overseas, through who knows what customs, etc. is a good question. Perhaps someone on this list lives near where some grows and can slip a few into an envelope to you? And I would bet that someone, somewhere in Europe (assuming that's where you are) is growing it, even if it's in a botanical garden. Finding it might be the challenge. I've pilfered many a seed, and cuttings also, from such places. Permaculture to me means useful plants worldwide are the property of the people!