Japanese SNLF units were essentially a larger modern equivalent of naval landing parties that navies had made for centuries. The naval landing party was typically made up of sailors who were armed and equipped as infantry in a pinch.
By the 19th Century naval landing parties were mainly sailors who supported marines with heavy weapons. At first, cannon as artillery ashore. Later, they manned machineguns and mortars supporting the marine infantry.
As the IJN had no tradition of marines, they orgainzed SNLF from sailors entirely. Early in the war these units resembled IJA infantry battalions. As the war shifted against the Japanese the SNLFs became tailored to being defensive units equipped primarily with heavy weapons. A typical late war one manned a number of coastal defense guns, machineguns and, mortars. They were primarily intended at that point to stop the landing and if that failed support the IJA infantry on an island.
Command of the SNLF was always seperate from IJA forces and usually poorly coordinated. This was due to an intense rivalry between the two services.