A water lily would be difficult to sanitize because the root system is so extensive. You'd probably have to harvest the bulb, cut off most of the roots and stems (maybe leave a few leaves and short roots to help it get going again), and let it regrow in your tank.
There are various ways to sanitize plants with bleach or peroxide; I've tried but I probably haven't done it right because I usually end up severely damaging the plants. If you're a patient person, you could simply rinse off the mud and quarantine the plant for two to four weeks in clean water. This will usually kill off any fish pathogens and parasites (because they can't find a host and die of starvation), allow any larval water bugs to grow up and move out, and give you plenty of time to find and squash any snails that might be lurking. If your water is chlorinated, so much the better. It isn't a perfect method but it would probably work.