Gartheia is one of the more secure pirate islands in the south sea. It's shape is similar to an immense flower pot - there are no decent ports or harbors around the circumference of the island as the stone cliffs overhang the waters lower then mast-height. In order to gain access to the higher parts of the island requires similar strategies to scaling a castle wall, with the disadvantages of having to do it from a rocking ship.
The pirates of the island have built, at great difficulty, a set of hoists and derricks to load and unload their ships. There are even plans to build one large enough to lift smaller ships completely out of the water, but these are still just plans.
Once on the upper portion of the island, it appears similar to many other islands in the region, albeit rocky.
The island itself is a massive block of sandstone which is gradually rising from the ocean due to geological forces. The actions of tides and normal waves are eroding the sides of the island as it rises.
See the Hopewell Rocks on Wikipedia. It has an illustration of the idea - the real rocks are much smaller then what I'm putting forth here. Yes this is a shameless plug of my home province.
If thrown into a moat or a ditch, they react with water to make an area of ground a foot wide and solid enough to walk on rise from the bottom. If eaten-well, the body of the one who ate one of them is a real mess. Used to storm moats and forge rivers, streams and ditches. Must be kept dry until used.