We have a large pond, which has comets, sturgeon and rudd as well as frogs etc. We were told koi will wreck plants. I've read that at least a third of the pond surface should have plants...partly for the fish to hide under, and partly to stop the sun turning the water green. Water lilies are great as they cover a good area, and look nice, but need quite deep, still water. Some kind of reeds around the sides are also good, as they add a bit of height.
As amonty said though, a lot depends on the size of your pond.