How do you enter Saronia Catacombs?

[Girlbeard]

So I've been trying to get in to this dungeon but I just can't find the entrance. I've looked up some videos and tried going to those exact spots (south of Southeast Saronia) but the Nautilus just won't enter the catacombs. Is there some other requirement? I've only recently gotten the ship so perhaps I've just being premature and need to wait for an upgrade?

[MegaNuke]

You have to enter by having your airship being able to turn into a submarine when you land on water and entering from the underwater cave. You have to talk to some people in one of the side towers in Saronia Castle after visiting Doga (if I remember correctly) and they will give your Nautilus airship underwater capabilities. Doga's manor is on the Southern Continent of the world map. Do everything there and then return to Saronia to get your ship upgrade.