I would wonder if the dissolved oxygen (DO) would be enough to support roots? When plants are grown with submerged roots (DWC / deep water culture) great lengths are taken to ensure the water is saturated with oxgyen using pond/tank aerators. In DWC you can never have too much oxygen.

How would you deliver nutrients to the plants? Tomatoes are heavy feeders and it's unlikely there is enough nutes in the pond water to support tomato plants.

--chris

The idea of having some tomatoes growing in a pond is interesting - I was wondering already if such thing is possible, perhaps with other plants. In other words I was thinking of it, but not considering tomatoes.

The oxygen probem could be solved by adding a airpump (stone) near- or directly under the plants (roots)- quite easily, if there are only a couple. The major problem might be the content of fertilizer, amount of makro nutrients and especially calzium. Even if the crops are not big, if they get Blossom End Rot, that would be no fun, right?

There also is the problem of support or fixing them - therefor I would rather chose some smaller variety, even a dwarf variety. Why not floating on some (round) styroform support! ;-)