You are probably too hot for watercress but you might want to grow it in the winter in the shady area. You can also get Sagittaria sagittifolia roots in some asian grocery stores. It is not supposed to be cultivated as it is an invasive species. I bought 2 tubers a few years ago and now have them growing in my pond. It is very similar to our Sagittaria latifolia (duck potato) but larger and with larger tubers.

You can grow tomatoes in water, but here in Phoenix just for a couple of months in the spring and a couple of months in the fall. Here's the link to my experiemnt at the Valley Permaculture Alliance web site.

There was also a thread about growing shrimp and Tilapia which you can get live at some Asian markets.

A rice farmer in Lissie, Texas, had a successful hydroponic tomato operation back in the 70s. About 10 green houses full and they were delicious. Much better than the ones picked too early for importing to the US.

You would need to buy some bagged topsoil to pot your aquatic plants in. It is cheap at big box stores.

Many tropical and hardy aquatic plants thrive in Phoenix, I had a ponding friend there who had a business selling off her extras as they grew so fast. You didn't say you were in Phoenix that is just where I used to live.

You can grow plants from tubers at the Asian market. You can also get them from other ponders, a lot cheaper than retail. There is an exchange as part of this forum and there are others too if you google. Many allow plants to be sold but this one does not.

Many aquatic plants are fast growing and small starts are fine.

There are not so many that are suitable for a home gardener to grow for food production. Most are ornamental.