There are total 7 spots in Ross Sea you have to reach to reveal the entire map. You need Explorer's Compass to know your current location in Ross and Margoria. Fishing Boat is good enough to travel around in Ross Sea, but it becomes way tougher in Margoria due to the hazardous environment. I bought the swimming suit and traveled around the sea just by swimming.

Below are the screenshots of the map around Ross Sea. The numbers labeled on each spot indicate the order I reached those points. I set off from Teyamal Island and traveled northeast.

Have a closer look at each spot. I took the screenshot as soon as the "You discovered a new region" message popped up.