Actually they are native to Florida, as are water lilies.
They were on the menu of the natives of Florida for thousands of years and their bones are found in the archaeological record.
As it happens, they do like to eat nonnative water hyacinth, which is an invasive species you probably confused with water lilies, but manatees don’t really have much access to freshwater ponds and lakes where hyacinth and waterlilies are usually found unless a flood puts the manatees there, since they cannot crawl overland. After big storms there are always some manatees and dolphins that need rescued and taken back to the rivers or sea.
My mistake. Apologies.