In new jersey ,we have a lilly pad which the pads are 4" and the flowers are white/ they are common. at 30" deep, would these, in your prediction, be the type that might cover 4 feet of water surface per plant?

That sounds about right (4 square feet surface area for a small/medium lily), if the waterlilies are kept in pots. The exact size depends on the conditions of the pond. If put into a bog or natural pond, they can of course spread.