This complex-looking plant has large flat leaves, then lacy-looking fronts resembling antlers at the end of air-filled stems. Smaller forms of this plant are used in aquaria.
This was floating in a lagoon on the north side of the Amazon River in Peru, bobbing on the water surface where I scooped it up for a better look. The roots were just hanging down in the water, and one frond was unfurling as a typical fiddlehead -- a tip-off as to its identification, as it doesn't really look much like a fern...
It's a very pretty plant, but like water hyacinth, it can take over a body of water. Here it was growing alongside water hyacinth, which seriously impeded the passage of our dugout canoe. People apparently used to eat it, but then they discovered it is carcinogenic when eaten. oops!