It's not a bad idea, but you may want to check on what they're finished with. Since they're exterior doors, they're built to handle moisture for quite a while, but you never know what they may be treated with. Since moisture is likely to condense on the underside and drip back into the pond, that will be your main source of concern.
I like the idea though, and it even comes with a latch so nobody can lift the lid from inside (even if they could lift something that heavy.
You could also go with an unfinished wood screen door, and then finish it yourself with either polyurethane or Krylon Fusion paint. That way you would know for sure that it was safe for the fish pond.
