If you haven't seen the Western Sea by Gentle Tractor or the Eastern Sea by me, you can find them here:
This map is far less "Fleshed out" than those two, as it lacks detail or map names. It's more of a proof of concept, showing how Sailing could deal with a cluttered and small map world.
To sum up: The Southern Sea is cramped and small. Jmods have said that a way to fix this would be to make the gameworld "unscaled" for sailing. (The gameworld is scaled down according to lore). So you'd embark from Catherby, and once you have the distance between Catherby and Karamja becomes much larger. It wouldn't affect the world map or any regular gameplay. You'd still be able to see people on the shores of the islands too. It's basically just stretching the sea out to make room for new stuff.
This is what the current Southern Sea looks like. It's a bit cluttered