Multiple Questions on Pirates (including treasures, monsters and etc)

Since after watching the Pirates of Caribbean, I would want to write a pirate story. However, I have met some questions on the way and I hope someone can give me some ideas:

• World structure: A cube-like world or a Flat World? I think having a cube like world can increase its space for including more adventures, but I also think that having a Flat world can do the same job but also having to connect the world in the same plane (since we can only afford to have one or two kingdom on one surface if we choose the cube-like world).

• Treasures: I want to build a legend of the Seven Mystical Loots, which is said to grant godlike powers to the owner. But the truth is they are sealed vessels of the sea monster of the ancient past. The problem is, I am short on treasures / artifacts, can someone give me ideas? I am thinking that the artifacts are corresponding to the sea monsters...

• Sea monsters: As mentioned above, the loots are sealed sea monsters. I want seven fearsome sea monsters that the pirates and merchants on the sea would fear. Currently I am on Kraken, Hydra... But I am not sure the difference between Cthulhu, Leviathan and Kraken? Should I go for the Lovecraft bestiary for these very fearsome monsters?

World Structure: As long as the entire world isn't explored and charted, I don't think worrying about the shape of the world is necessary. In fact, it might increase the sense of adventure if it isn't.

Treasures: There's coins, masks, crowns, scepters, swords... Heck, any object that could possibly exist in the setting.

Sea monsters: Cthulhu is basically a humanoid with an octopus head and bat wings, I think Leviathan was originally some sort of giant sea snake/dragon, and Kraken is a huge-ass cephalopod. Let your imagination run wild here, everything huge and swimming can serve as a sea monster. Giant lobster? Giant fish? Giant lobster-fish? The possibilities are endless.

One of Terry Pratchett's Discworld novels described what happens if you get too close to the edge. There is another continent on the underside that acts as a counterweight, but most Discworld inhabitants (of the upper surface) think it's just a myth.