Cygillan Ocean is a body of water in the world of Eos in Final Fantasy XV. It is the ocean south of the continent of Lucis, and Accordo Protectorate exists as a series of islands within it. The sea god Bismarck is said to dwell in its depths. The ocean is ripe with fish. The number of boats sailing it has been reduced after Niflheim began using airships as its main means of transportation.
In the original version, the player crosses the ocean on the royal vessel at the start of Chapter 9. The ocean-cross is an in-game cut scene, during which the party will converse about what is going on in the rest of the world. In the Windows and Royal Editions, Cid Sophiar gifts Prince Noctis the key to the royal vessel as they arrive in Altissia, and the player can take the boat out to a cruise after talking to Weskham Armaugh at the Maagho. The player can manually steer the ship and dock at Altissia, Cape Caem and Galdin Quay. They can fish from the vessel, look for Bismarck, and take photos.
Fishing spots are denoted on a radar and can keep changing. When sailing, fishing-related quests become available when the player approaches the correct spot. The ocean only has two unique fish: the Mummy Bass (fished from the spot Gladiolus suggests, near Angelgard, by using another hooked fish as bait) and the Styrial Bluefin Tuna, found on the spot where Bismarck appears. Mummy Fish is the hardest fish to reel in, but yields a good item for Elemancy when caught. Catching Styrial Bluefin Tuna teaches Ignis a superb recipe, which is one of Noctis's favorites.
Cygillan Ocean by the islands en route to Altissia
Sail the royal vessel manually
When sailing the royal vessel, Ignis speaks up of a fishing spot near the islands en route to Altissia. Sailing onto the spot completes the mission. The player can fish there and catch a Nephilim Grouper.
When sailing the royal vessel, Gladiolus speaks up of a fishing spot near Angelgard. Sailing onto the spot completes the mission. The player can fish the rare Mummy Bass there by hooking a fish and not reeling it, using the fish itself as the bait.
Mummy Bass is the hardest fish to catch. The player should have the ultimate fishing gear and use Dragon's Beard line. Eating Kupoberry Cheesecake also helps. One trick to reeling the fish in is to tap the pause button and adjust the reel/do not reel as needed, and always reel to the direction the fish swims, even before the prompts appear on-screen.