Freshwater and brackish wetlands and marine mangroves are not a compact group of ecosystems. Nevertheless it is useful to put them together just for purposes of this website. All of them are characterized by high biodiversity. They are extremely important for the landscape ecology.
Wetlands are considered the most biologically diverse of all ecosystems. Plant life found in wetlands includes mangrove, water lilies, cattails, sedges, tamarack, black spruce, cypress, gum, and many others. Animal life includes many different amphibians, reptiles, birds, and furbearers.
Wetlands have also been described as ecotones, providing a transition between dry land and water bodies.
A swamp is a wetland featuring temporary or permanent inundation of large areas of land, by shallow bodies of water. A swamp generally has a substantial number of hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types of swamp are "true" or forest swamps and "transitional" or shrub swamps. The water of a swamp may be either fresh water or salt water. A swamp is also generally defined as having no substantial peat deposits, as opposed to a bog which accumulates peat.
In geography, a marsh, or morass, is a type of wetland which is subject to frequent or continuous flood. Typically the water is shallow and features grasses, rushes, reeds, typhas, sedges, and other herbaceous plants. Woody plants will be low-growing shrubs. A marsh is different from a swamp, which has a greater proportion of open water surface and may be deeper than a marsh. In North America, the term "swamp" is used for wetland dominated by trees rather than grasses and low herbs.
The water of a marsh can be fresh (freshwater marsh), brackish (brackish marsh), or saline (salt marsh).
Coastal marshes may be associated with estuaries, and are also along waterways between coastal barrier islands and the inner coast. The estuarine marsh, or tidal marsh, is often based on soils consisting of sandy bottoms or bay muds. An example is the Tantramar Marsh of eastern Canada.
Marshes are critically important wildlife habitat, often serving as breeding grounds for a wide variety of animal life, particularly including ducks and geese.
A bog or mire is a wetland type that accumulates acidic peat, a deposit of dead plant material—usually mosses, but also lichens in Arctic climates.
Bogs occur where the water at the ground surface is acidic, either from acidic ground water, or where water is derived entirely from precipitation, when they are termed ombrotrophic (rain-fed). Water flowing out of bogs has a characteristic brown color, from dissolved peat tannins. Bogs are very sensitive habitats, of high importance for biodiversity.
Mangroves (generally) are trees and shrubs that grow in saline (brackish) coastal habitats in the 
tropics and subtropics. The word is used in at least three senses: (1) most broadly to refer to the habitat and entire plant assemblage or mangal, for which the terms mangrove swamp and mangrove forest are also used, (2) to refer to all trees and large shrubs in the mangal, and (3) narrowly to refer to the mangrove family of plants, the Rhizophoraceae, or even more specifically just to mangrove trees of the genus Rhizophora. Mangals are found in depositional coastal environments where fine sediments, often with high organic content, collect in areas protected from high energy wave action.
Mangroves support unique ecosystems, especially on their intricate root systems. The mesh of mangrove roots produces a quiet marine region for many young organisms. In areas where roots are permanently submerged, they may host a wide variety of organisms, including algae, barnacles, oysters, sponges, and bryozoans, which all require a hard substratum for anchoring while they filter feed. Shrimps and mud lobsters use the muddy bottom as their home. Mangrove crabs improve the nutritional quality of the mangal muds for other bottom feeders by mulching the mangrove leaves. In at least some cases, export of carbon fixed in mangroves is important in coastal food webs. The habitats also host several commercially important species of fish and crustaceans. In Vietnam, Thailand, the Philippines, and India, mangrove plantations are grown in coastal regions for the benefits they provide to coastal fisheries and other uses. Despite replanting programs, over half of the world's mangroves have been lost in recent times.
Albatros Travel and Expeditions can provide you many trips directed into areas with important wetlands or mangroves worldwide. These trips are usually easy. Even not trained people can take part in such journeys. There is a list of countries where these ecosystems can be visited:
Indonesia - national parks at almost all important Indonesian islands.
Sri Lanka - national parks along east, southeast and northwest coast
Tanzania - wetland in many inland national parks, mangroves on the eastern coast
New Zealand - quite rarely, only one national park in each island
Chile - wetlands in several parks scattered within the whole territory
Ecuador - mangroves along the western coast and in Galapagaos, wetlands in Amazonia and in Andes