Lilly pads float; they have air sacks to help them do so. In order to get the pad to stay submerged, you would have to put a weight on the pad that would make it sink.

I am guessing that what you meant to ask was:
How does a lily pad grow in water?

There are a lot of different species of water lilies but for the most part lily pads float on the water and have roots that reach the muddy bottom. Lily pads get nutrients from these roots just like other plants. Some lily pads can also absorb nutrients from the water itself via either roots just dangling in the water and perhaps even via the leaves directly. Of course all lily pads also photosynthesize.