I have recently been researching a similar situation re one of my ancestors, also a mariner, but in Kent. This text, from a document published by the Kent Archaeological Field School, helps explain the number of mariners in 18th century England. Although the document itself is primarily about the development of Faversham, this paragraph refers to the use of waterways and coastal traffic around England due to the general lack of passable roads for freight traffic. I assume that, besides the fishing trade, many mariners would have been engaged in general goods transport akin to the jobs of train and lorry drivers today. So it was likely to have been a fairly common job amongst people living near the coast and waterway networks.
“Throughout the 18th century the principle highway of England was the sea. Before canals or railways, and while roads remained impassable, coastal shipping remained the cheapest, safest and speediest means of conveying freight. Hence ports were vital, not just for trade, but also as nurseries of the Royal Navy, the fisheries, and the whaling fleets. All of the front-rank towns of the kingdom were either ports or had easy river access to the sea (Selley 1962: 199). Besides the ships of the Royal Navy and merchantmen trading overseas, there were large numbers of small craft trafficking in the waters about Britain. “There are supposed to be about eighteen hundred ships and vessels in the coal trade and about nine hundred more in what they call the Northern trade”, wrote a naval officer in 1774 (Ashton, 1924: 200). North Kent was endowed with one passable road (Watling Street) and numerous waterways. It possessed an extensive coastline along the Thames south shore and to the east, a navigable river from Sandwich to Fordwich (and thence to Canterbury), and to the west the Medway River from Rochester to Maidstone. Fortuitously “the water transport was available where it was most needed” (Thirsk 1967: 199). Contemporaries were aware of this favourable circumstance, and noted particularly “the benefits of water carriage (from Kent) to and from the Metropolitical City, or Chief Mart” (Harris, 1719: 357). Throughout the year coastal hoys operated a weekly schedule from these North Kent ports.”