The top four commercial sources are mainly porcine skin/bone and bovine hide/bone. Marine sourced gelatin (e.g. fish) is only produced and applied on a small scale.

However, with religious or ethical reasons from pig and beef gelatin, as well as diseases such as BSE (Bovine spongiform encephalopathy) and foot-and-mouth disease, fish gelatin can be a replacement.

Porcine gelatin
It is made from pig skins or bones. Pigskin based one is the most commercially used gelatin in the world owing to its cheaper price. However, its applications are restricted in Islamic countries as it is not halal.

Bovine gelatin
It is made from bovine hides or bones, which can be accepted as kosher and halal, complies with the law of Muslims and Jews, but not vegan friendly.

Fish gelatin
The parts of farmed & fresh fish (e.g. skin, bones, scales, and fins) can be used to produce fish gelatin. These fish species include cold-water fish such as alaska cod, pacific cod, green pollock, and salmon; and warm water fish such as tilapia, grass carp, squid, and tuna.

Versus porcine and bovine gelatin
With the similar functionalities of gelling, foaming, emulsifying and binding, fish gelatin can be used as a substitute for bovine and porcine gelatin with the following two advantages:

Acceptable for all religions and cultures and is almost suitable for all consumers.
Without the risks of disease that would happen in bovine and porcine.  
Compared with pig and bovine gelatin, fish gelatin has lower molecular weight, lower gel strength and lower melting temperature, resulting in lower market demand.

But this is a unique property in some specific applications. For example, fish gelatin confectionery melts faster at a higher temperature and releases its flavor more quickly in your mouth, which gives you a flavorful feeling. So fish gelatin is suitable to be used in desserts which needs a low melting point and quick flavor release in the mouth.

Meanwhile, fish gelatin has a more or less fishy smell, and this smell limits its application.

Imino acids

Generally speaking, compared with mammal sources, gelatin derived from aquatic animals has poor gel properties is mainly due to its much lower imino acid content (proline and hydroxyproline), especially from cold-water fish. The higher the content of imino acids in gelatin, the better the gel properties.

Chicken gelatin
Chicken gelatin has a high gel strength, commonly made from chicken skin or bones, which is rich in collagen.