Let's come up with an amphibian race!

If the underground sea is not connected to the surface, then the inconsistency disappears. You can descend into the underworld through passages in the Dungeon. Movement there will be possible thanks to the skills acquired for underwater walking. So, everything seems to be fine here.

No. You didn't understand. I'm talking about the fact that it would be logical if the underwater race could move through the sea (because they can also collect chests on the water) in the same way as underwater, i.e., without a ship. And other races move across the surface sea on ships.