I just got back from a trip to Longwood Gardens, in PA. They have a pond in
the entrance to the new arboretum with a safety grid made of structural
plastic. It as plates about 3' square, and brackets that link them all
together. It is placed just a few inches below water level. While viewing
the pond from an angle, and at any resonable distance, the grid is not too
noticable. It is made with a grey color opaque plastic though, and therefore
is very noticable at close distance, or directly from above. I have also
seen water features made using transparent acrylic tubing, in which the
tubing is completely invisible. Ripley's museum uses this technique in
making their perpetual floating faucet that apears to float in mid air, with
an eternal stream of water flowing from it. In reality, the water is pumped
up the inside of the "stream" within an acrylic tube, and then flows outside
the acrylic tube to apear as a solid stream of water coming from nowhere. If
this material were shaped into grid of sufficient thickness, and place with
proper bracing and support, it would achieve the result you are looking for.
It would be completely invisible, and structurally strong for safety.
The down side of this approach would be: 1) The material you would need
would be incredibly expensive for the thickness you would need. 2) The cost
of fabrication for the components would be terribly expensive. 3) The burden
of maintenance would be outrageous, as the pristine water of an uninhabited
pond would be much different than a Koi pond with the attendant nutrients,
algea and micro-organisms.
You may want to consider getting serious estimates for the expenses involved
before you begin digging. Last I checked, the cost of a sheet of 1" thick
acrylic was in the thousands of dollars. Then this price of cutting the grid
is another story all together. Hope this helps with the dream pond idea
though. You never know when your going to hit the Big Lottery! That
should help pay the staff that you would need to keep it all pristinely
clean.
Anything is possible. Being willing to comit the resources is something else
all together.
On the other hand, if you are willing to accept the blocked view of the
opaque plastic, just contact Longwood Gardens for information on where they
obtained the commercially available materials they used. I'm sure it would
be much more reasonable than my solution, even if it's not completely
invisible!

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The information offered in this post is offered in the spirit of trying to
help. This is in no way a claim to be an authority of any kind on any
subject.