An odd naval Dirk with a shell guard and what looks like a triangular blade, British or American, ca. 1800-1820, from the Curator’s Eye Antiques.
I again invoke the knowledge of @victoriansword.
British naval dirks were not regulated until the Victorian period. In the 18th century and early 19th century pretty much anything was an option. There was a great diversity in designs for naval dirks; broad and double-edged, thin and double-edged, curved with one edge, and blades with thrust-oriented cross-sections such as diamond, lenticular/oval, square, rectangular, and less commonly, triangular cross-sections. They also varied in length. Dirks had various grips including ivory, ebony, shagreen, and brass. There were various pommel designs, and quite a few guard types, as well. Guards might be simple quillons, a disk, and knuckbow, quillons with a decorative chain attached to the pommel…. Anything goes when it comes to naval dirks of the Georgian period.
Above: A variety of British naval dirks with straight double edged blades. (source)
Above: A curved and single-edged British naval dirk and a small double-edged naval dirk. (source)
Above: Three British dirks of diverse form. (source)
Above: Here is a British dirk with a wide lenticular cross-section, and it looks similar to a ballock dagger. (source)
Slightly later in the 19th century, in 1833, the French Navy adopted a dirk/dagger with a short blade of triangular cross-section.
Above: French M1833 boarding dagger/dirk (source)
So, yeah, British (and American) naval dirks were pretty diverse in just about every aspect of design.
Check out the online collections of the National Maritime Museum in Greenwich to see their large collection of naval dirks.
Note that these last ones were issued to French troops in WW1 around 1915 when trench warfare was being developed but before they pulled cutlers from the frontline.
@carminegalleon dirks are basically just regular daggers but For Sailors