"Waterside" listed as occupation in England during the 1820s
When I look up that term now it seems to be a stevedore - but the man in question would have been 60-70 years old and it seems unlikely that he would have been one.
The location is an agricultural area of Lincolnshire, specifically Billinghay or Sleaford. The only things I could see being shipped would be grains or meat. In the city directory where I found this occupation listing it seems to be under the heading of "Maltster" and a later listing shows someone with the same name as proprietor of an inn or tavern named the Barter's Arms.
A letter written by his son in 1832 indicates that the father and most of the family are ailing from typhus. It was in response to a letter from my great great grandmother (his daughter who had married unsuitably and who moved to New York state in 1828) after her husband and one child had died, his mill burned down and she was left with six young children and no resources. The letter does discuss plowing and the price of oats which fits with the farming referred to in the following:
Family lore is that "Greatgrandfather Key had six sons and four daughters. He and each of his sons stood at least 6 feet in their stockings. At meals they drank home brewed ale and each one had a sterling silver tankard (a cup with two handles). Grandmother said her father wore knee breeches, black silk stockings and had shoes with silver buckles; and she never knew him to do a days work in her life. He was a farmer on a large scale with many tenants. His sons worked." (From a letter written by my grandmother's cousin.)