Thought this might be of interest. I was in southeastern Peru this fall and just posted two new biotope profiles, which include some underwater pictures of plants. There's a general overview at http://news.mongabay.com/2005/1127-biotope.html
Amazon oxbow lake biotope
Virtually all lakes in this part of the Amazon are oxbow lakes. An oxbow lake is a crescent-shaped lake formed when a river changes course. In lowland Amazonia where soft alluvial soils dominate, meandering rivers gradually shift due to erosion and sediment deposition. Oxbow lakes typically form when loops in the river become so extreme that the main channel erodes a new straighter route, leaving the river bend apart from the river. As time passes, the oxbow lake becomes increasingly distant from the main channel. Water conditions change as the water stagnates.
Oxbow lakes in this region have considerable aquatic plant growth. Most conspicuous are free-floating plants including Eichhornia, Pistia, Salvinia, Victoria cruziana, and Bladderwort (Utricularia foliosa), which grows densely and extensively in the lakes. Amazon sword plants (Echinodorus sp.), Vallisneria, Panicum, and Hymenachne were also present, though in lesser numbers.
Amazon stream biotope
Small rainforest streams are common in this part of Peru. They generally arise from springs and range in width from a few inches to 10-14 feet according to Amazon Headwaters. Rainforest streams in the region rarely exceed 3 miles (5 km) in length.
I observed very few aquatic plants in the rainforest stream -- especially in shaded areas where very little light penetrate the dense canopy. In a unshaded pool section an Amazon sword plant (Echinodorus sp.) was present along with a grassy Vallisneria-like plant. There were plants growing with roots submerged but leaves emersed in some sections of the stream