The following are some less well known perennial edible aquatic species for hardiness zone 9. I see them as less well known as they are not listed in the best single source on aquatic edibles by far, the book "Edible Water Gardens" by Nick Romanowski.
Apium nodiflorum. "Sedanina" "Fool's Watercress". This is an outstanding vegetable in flavor and productivity, it is even reasonably productive throughout zone 9 winter. The leaves and leaf stems are edible, tasting much like celery. It spreads vegetatively in the water and new leaves quickly replace picked ones. In my experience above ground parts die off after setting seed so there is a lull in productivity in summer, but new growth emerges from the soil shortly after, I can't tell if this new growth is from new plants or the same plant. Perhaps overlooked in many places because of it's unfortunate misleading name "Fool's Watercress", it is better than watercress from my point of view, easier to grow, not needing running water, grows very well in still water. It will grow in full sun or considerable shade, much more compact growth and smaller leaves in full sun, seems to be best in semi-shade. Some authorities in New Zealand are concerned about this plant as a weed, hopefully they don't ban this excellent crop in their probably futile crusade to protect natives. References: Wild edible fool's watercress, a potential crop with nutraceutical properties. Carla Guijarro-Real et al 2019. Ethnobotanical and Ecological Studies of Wild Edible Plants in Jordan. Saleh A. Al-Qura’n 2010.
Rotala rotundifolia. This plant is eaten in India (potherb) and China ( "Wild edible plants collected by Hani from terraced rice paddy ecosystem in Honghe Protectorate, Yunnan, China." Luo et al. 2019. Ethnobotany of Rewalsar Himalaya. Sood & Thakur 2004). The leaves & stem are edible, I also eat the tiny white roots that grow along the stem. I eat it uncooked. It is basically bland in flavor but it's crimson coloring is very ornamental in a dish. It is of good productivity in summer. It does not die back in winter, but there is no new growth. The plant is common in the aquarium trade.
Colocasia fallax grows rather slowly in water in temperate climates, a bit too slow to be of much use I find. There are very few references to it being eaten, the tubers, stem and leaves have been eaten (after cooking presumably) in India (An ethnobotanical note on wild edible plants of Upper Eastern Himalaya, India, G Murtem and P Chaudhry. 2016.) and the stems are a famine food in China (Food Plants of China. Shiu-ying Hu. 2005.)
I have not gotten around to eating it, I expect it is like taro in flavor and extensive cooking-time requirement to eliminate toxins. The leaves do not die down in zone 9 winter as taro leaves do. It also spreads slowly.
I'm pretty certain this plant is the New Zealand native Myriophyllum propinquum
( https://www.nzpcn.org.nz/flora/species/myriophyllum-propinquum/ ). It was sold as Hippuris vulgaris, which it resembles, by a New Zealand aquarium supplier, which is edible but prohibited in New Zealand. Believing it was what they said it was I ate it quite often before realizing it had been misidentified. I have continued to eat it, even though there is no record of it ever being eaten anywhere, because it is bland in flavor, which almost always means something is safe to eat and because I have noticed no ill effects and because several other Myriophyllum are eaten overseas and none are known to be toxic.
I thought the tiny leaves might be a choking hazard but this has not been the case. It is reasonably productive and younger growth has a pleasant enough texture, older grow is a bit tough and chewy, I suppose it is a bit visually interesting or novel in a dish.
Cardamine lyrata. "Japanese Cress". The leaves and (numerous but tiny and threadlike) roots are edible https://pfaf.org/user/Plant.aspx?LatinName=Cardamine+lyrata
Unfortunately the bland tasting leaves are pretty small, usually less than 1 cm wide, it is unusually time consuming harvesting them, it is tempting to try to eat a bunch along with the stems but the stems are usually too tough to eat. It also spreads very aggressively horizontally across the surface of the water smothering other plants. Aside from these horizontal shoots it is not very productive. I find it more of a problem than anything. I keep it in a tub a long way away from other water plants and rarely harvest it.
This plant is available through the aquarium trade.
Cryptotaenia japonica, mitsuba. There only seems to be one reference to this outstanding vegetable growing in water; Cornucopia II by Stephen Facciola states the Kansai variety grows in water. I tried what is probably this variety, I found it does survive and set seed in water in but grows extremely slowly and remains very small, almost completely unproductive. But this vegetable is so valuable it probably deserves further trials.
Filipendula ulmaria "meadowsweet" grows well in shallow water. The leaves are often reported to be edible when young, I find them a bit too chewy and dry but they have a pleasant interesting "root beer" flavor due to coumerin content. The roots are also rarely reported to be edible
https://pfaf.org/user/Plant.aspx?LatinName=Filipendula+ulmaria
but don't seem to have been described anywhere,
they are about the width of a pencil, reasonably long, fiber free, crunchy and have a very pleasant distinct "root beer" flavor, even stronger than the leaves. I've only eaten them raw. The roots don't appear to be produced in great abundance but I have not been growing it long enough to be sure, they definitely deserve further research. Since the plant is clump forming it should be possible to harvest roots from part of the clump while leaving the rest to grow for continuous future harvest. The flowers are also edible, I have not tried them yet. Even the seeds have been reported to be edible.
https://gallowaywildfoods.com/meadowsweet-identification-distribution-edibility/
Filipendula kamtschatica also reportedly grows in shallow water, and is edible. It does not seem to be readily available, I've not tried it.
Bacopa caroliniana (center of photo) is also aquatic and edible
https://edis.ifas.ufl.edu/publication/AG392
Leaves and stems have a pleasant flavor unlike the more well known Bacopa monnieri (the most bitter thing called "food" I've ever tasted). I've found it to be productive in the height of summer provided it does not have competition from more aggressive plants.
I found this plant in the aquarium trade.
Callitriche stagnalis, Water starwort. Shui-ma-chi. The whole plant is, or was, eaten in Taiwan (Food Plants of China. Shui-ying Hu. 2005). It is a weed in New Zealand, it does not seem to be available commercially. I have not grown it for a while, I don't remember anything very distinctive about its favor, I found it reasonably productive. It can grow floating in the water and only grows about 1 cm above the surface.
I also tried Lindernia rotundifolia, the tender leaves of which are eaten in India (Ethnic uses of some wetland plants by the Bodo community in Udalgiri district of Assam, India. Boro & sarma 2013.) Despite a report that it is cold tolerant it did not survive the winter here in zone 9.
