Mermaid Weed (Proserpinaca palustris)
Source: Go Botany Native Plant Trust
Mermaid weed grows in the submersed and emergent plant communities. It may be found in the shallow waters of bogs, marshes, swamps, and along the muddy shores and banks of ponds and streams. The plants are well adapted to life at the water's edge and may morph to a terrestrial form when water levels drop. (Maine Field Guide to Invasive Aquatic Plants)
These plants are noted for their variety of leaf forms on the same plant. Submerged leaves are deeply divided while the emergent leaves are not divided but do have serrations. If emergent leaves are not present this can resemble Milfoils which are members of the same family. The key difference is the arrangement of leaves—Mermaid Weed leaves are alternate and not whorled. (A Guide to Aquatic Plants in Massachusetts).