The typical smell of (dead) fish is based on a chemical compound called trimethylamine (TMA).
Fish do use it in the regulation of their metabolism during pressure changes.
For humans there is a body odor disease known as trimethylaminuria (TMAU)
This desease has two types.
The first type is based in a defect of the FMO3 enzyme found in the human liver metabolism and people affected by that do emit a fishy odor.
The second type is different. Patients with the type 2 pattern show an overload of the FMO3 enzyme caused by a largely increased TMA synthesis in the small intestinal tract.
The research in the last decade assumed a kind of wrong type of bacteria in the gut, which would produce large amounts of TMA just by introducing them into the intestinal tract.
This assumption seems to be incorrect.
Todays view shifts currently. It is very likely that a permanent malabsorption of choline and betaine is the cause of the altered bacterial metabolism activity.
More interestingly most of the TMAU sufferers do have other dominant smell types than fishy. The most stated type of smell was a fecal smell.