Photos by Maria Gross. These samples were collected by Brad Gaudreault in South Cove near Boynton Island (lat/long: 43.941589N 71.062546W). Water marigold, is a native and is characterized by branching leaves surrounding a stem.
The stems of water marigold emerge from buried rootstalks and rhizomes. Two distinct leaf types are formed. The submersed leaves are finely divided, and oppositely arranged on the stem. When preparing to flower, lance-shaped leaves with serrated margins emerge from the surface of the water on robust stalks. The emergent leaves are also oppositely arranged and attached directly to the stem. Showy, yellow, daisy-like flowers (2 to 2.5 cm wide) are produced among the emergent leaves. (Excerpt from Maine's Interactive Field Guide to Aquatic Invaders).
Look Alikes: Coontail is similar, but does not branch as much, and leaves cup upward and feel plastic/course. Fanwort is also similar, but the fan shaped branching leaves are opposite, and in small stems off the main stem.