Well I have experimented with ponds slightly as well as outdoor
tanks in the summer months.  So here are some things to consider from
my experience.
        1/ ponds get big fast.  A simple 4 foot wide by 12 foot long and
3 feet deep pond is a small pond but just total up its gallonage.  You will
find that even for something this small if you want CO2 it is huge.
        2/ light from the natural sun is both more benificial and more intense
than what you can get from lights of any kind.  It is also harder to control
this light as you cannot control how many hours the sun will shine.
        3/ many plants will be killed by excessive sun, solution is to have 
them planted deeper so for crypts and the other lower light plants they need
to be planted deeper.
        4/Growing plants in tubs outdoors can be very rewarding once you get
the hang of it.  Four years ago I had 6 Tubs 4 foot by 3 foot by 18 inches
deep.  No CO2 or anything special each got about 4 plants in it is late April
and in September I sold plant to local Shops making over $600.00 US.

        Bad thing is where I live it is very seasonal.  There were some 
places that did it comercially around here but few survived as when your
doing it for a living you need a more steady cash flow.

        If your in a warmer winter climate where you know you can keep your
outdoor water temp well above the 70's F then you have some good potential 
there.  But in Cold Wisconsin you will need a heater system that will eat up
all your profits before you know it.  Or take all the fun out of it.
        If anyone wants more details on outdoor plants give me yell and we can
take this to personal E-Mail.