By plant cover, I presume you mean shade for fish and some protection from predators? The hardy waterlilies will do the job though you would need to crop them maybe every three or four years.
A hardy lily might put on three or four feet growth a year, at least they wont be invasive and tunnel uncontrollably into a lake bed like cat tails or lotus might.
A few good names to look for among the hardies , gladstonia, attraction, rembrandt, colorado, hollandia, pink sensation, sunrise are a few.
Try not to be tempted by the 'cheapo's' that are often thrown in a pond, its not a bad idea to put varieties in with good blooming characteristics as when you have a surplus, you will find the good ones fetch a reasonable price on e-bay, whereas plug plain whites like alba or a pink like carnea are less easy to get rid of
Regards, Andy
