I live in Cambridge, England, and the land around here is FEN. In fact, if you drive north about 15 miles you enter FENLAND - a part of the county that is farmland drained by a series of ditches, canals and pumping stations. This fenland extends north through Lincolnshire, and east into Norfolk, and west into the midlands. The countryside is very flat, it's dry farmland between the drainage ditches. It's agriculturally very productive with two crops a year in many parts. The crops are varied - potatoes, cabbage, asparagus, celery, and ornamental bulbs and flowers. With so much water around there are a lot of birds, particularly swans and ducks. In the west of England and the north, and in Wales and Scotland, you get MOORLAND. This often consists of hills covered in heather, gorse, and sheep, and deer. These upland areas get very boggy due to the rainfall and underlying impervious rock. Moorland can be rather flat, but it looks nothing like the low-lying fens with its drainage network and pumping stations. Much of Fenland is below sea-level and at risk of major flooding from the sea. Much moorland is well above sea-level and at no risk of flooding. They really are very different. We have ground that is rather 'swampy' but in England we don't have large areas of swamp. And we don't have alligators cruising through them. We have the odd bog or two - which is a really swampy wet muddy area. They are small - some only a few yards across. You might find really dangerous bogs on the moors. Marshland is a rather mixed bag of types. Any rather soggy areas we might call 'marsh.' Where the FENS are less well drained you get MARSH. Marsh is NOT useful for agriculture, but is perfect for wading and nesting birds. So, around here there are very distinct types of land. Most is FEN. There are some MARSHES, particularly near the coast. We have no MOORS here - you need to go to the higher ground to find a real moor. So, I hope that helps, and goodbye from me here in FENLAND.

And to add a New Zealand context: We pretty much only use Swamp. Marsh is occasionally used, and to me sounds estuarine. Bog is a toilet, an Irish Bar or a verb. 'Mire' and 'fen' would be probably the least used of them.

The previous answers (bar one that was written as I wrote this) are from Americans who don't seem to have access to a dictionary, and were misled by your talk of false friends. To be fair, the wetter parts of moors are often covered in blanket bogs. Marshes are areas of waterlogged land near a lake, river or (in the case of a salt marsh) the sea. I think 'swamp' is synonymous with it, but it is not used locally in the UK, though we might use the term to describe exotic places such as mangrove swamps, which we can only imagine. The other terms all relate to different types of peatland, a geography found extensively in the UK, though heavily reduced by human activity. Most Londoners have never seen a bog, but most of the inhabitants of Donegal have, so the terms are used with varying degrees of accuracy. The 'correct' terms are defined here https://en.m.wikipedia.org/wiki/Mire and would be used by people who are aware of and interested in these types of landscape. In more general use, i.e. by people who don't live on or near peatlands or study them: Fen: Norfolk, Lincolnshire and Cambridgeshire are very flat, and were covered in extensive peat marshes until the 17th century, when they were drained for farming. People are referring to this area of drained marshes when they talk of 'the Fens' the actual fens have largely disappeared. Bog: used to describe unpleasant, waterlogged land, and as a synonym for toilet. Mire: used to describe a patch of deep, wet mud and/or dung.

A mire = a shortened word for "a quagmire" which is an area of soft wet land usually grassy that you sink into when you try to walk over it. The depth that you sink will range from less than an inch 25 centimetres to about 2 inches 50 centimetres, although it could be deeper. Any land or grassy area especially clay can quickly become a quagmire after heavy rain fall or prolonged rainfall. Quagmires are difficult and heavy to walk over/across they physically drain all your energy, because you have to constantly make strenuous efforts to pull each foot meticulously slowly and consciously back out of the "STICKY" heavy "clay" ground. From this comes the idiom "caught in a quagmire" used when you are in a difficult situation of any kind and used most commonly if you are in a difficult emotional turmoil or situation you cannot seem to drag yourself out of. A bog is not exactly the same it is usually wet peaty ground, or slang for toilet. Fen is fenland(s) to describe the Fens of Norfolk and Lincolnshire and areas on the East of England near the coast.

Shall we blame firstly "hollywood" for not making movies about crocodiles or alligators attacking people in the Bogs ,or fens but always only in the "everglades of Florida and Miami", or in general in swamps. Or shall we blame secondly the invaders of the British isles who made our language especially the regional differences, or shall we thirdly blame the migration patterns over to the USA US or the States (brits like the word America), or the migration and punishment pathways into Australia and the colonies. Shall we blame fourthly the migration patterns to and from the commonwealth.Or do we correctly blame all of these things collectively. I am a Londoner and I have heard the term swamp and swampy ground even as a child, before hollywood and movies and televisions or the internet, at a time when none of these existed in the average persons home. We could repeat this discussion for the hundreds of words used in the English language and across the globe for the names and terms used for "bread". Where researchers can and do track back your family roots.