I have a iris type plant that sits on bricks so that it is a few inches from the bottom.I have divided it one time,and even though I don't have the greenest of thumbs, I did get it to bloom once.
I also have some of the water hyacinth that floats on top. I think I gave about 3 dollars a couple of years ago for the iris, and the water hyacinth and floating types are about the same price range, but I have never been able to keep the floating ones over the winter.You can also place other potted plants around the pond and set them at various heights, or hang them from small shepherd hooks.
As for the mosquito issue, I haven't found it to be a big problem. The water evaporates pretty fast and just adding more water and sometimes dipping some out and replacing it usually works for me.