A Vision of Britain through Time provides the following description of Beesby in the Marsh from John Marius Wilson's Imperial Gazetteer of England and Wales of 1870-72:
Beesby in the Marsh was a parish in Louth Rural District from 1894 until 1974. In 1974 it was transferred to East Lindsey non-metropolitan district along with all the other parishes in Louth Rural District and the surrounding area. Since 1974 references are made to the civil parish of "Beesby with Saleby" with the comment that it used to be called Beesby in the Marsh. (Source: Wikipedia) It would appear that a modern civil parish has been formed from Beesby in the Marsh and the neighbouring parish to the south: Saleby with Thoresthorpe.
There is no mention of Beesby with Saleby in A Vision of Britain through Time where transfers of land between parishes are usually noted up to 1974.
Lincolnshire is very low-lying and land had to be drained for agriculture to be successful. The larger drainage channels, many of which are parallel to each other, became boundaries between parishes. Many parishes are long and thin for this reason.
There is much fenland in Lincolnshire, particularly in the Boston and Horncastle areas. Fenlands tended to be extraparochial before the mid 1850s, and although many sections were identified with names and given the title "civil parish", little information has been found about them. Many appear to be abolished in 1906, but the parish which adopts them is not given in A Vision of Britain through Time. Note the WR category Lincolnshire Fenland Settlements which is an attempt to organize them into one list.
From 1889 until 1974 Lincolnshire was divided into three administrative counties: Parts of Holland, Parts of Kesteven and Parts of Lindsey. These formal names do not fit with modern grammatical usage, but that is what they were, nonetheless. In 1974 the northern section of Lindsey, along with the East Riding of Yorkshire, became the short-lived county of Humberside. In 1996 Humberside was abolished and the area previously in Lincolnshire was made into the two "unitary authorities" of North Lincolnshire and North East Lincolnshire. The remainder of Lincolnshire was divided into "non-metropolitan districts" or "district municipalities" in 1974. Towns, villages and parishes are all listed under Lincolnshire, but the present-day districts are also given so that places in this large county can more easily be located and linked to their wider neighbourhoods. See the WR placepage Lincolnshire, England and the smaller divisions for further explanation.