In a fantasy world that I am working on, the world is covered in one massive ocean, with the only land being these 300 square miles "god" corpses (that do not decay) that can grant people magical abilities when they come in contact with a certain part of it. How this came to be is not important. What is important is that now humanity is constantly fighting for control of these bodies and their magic-giving properties.
Those not lucky enough to have to access these bodies (which is most of the population) live in giant, self-sustaining migratory ships about the size of a cruise ship. The people on the corpses make massive cities both on and inside the corpse, that, like the ships, are self-sustaining.
So, in order to get these corpses, the people must fight for them or at least be able to get inside of the corpse and perform a ritual in which the people performing it will achieve the magical ability that takes about thirty minutes.
So, with this in mind, how would a group of ships be able to invade one of these cities most effectively, and not destroy it in the prosses as they need to keep the corpse intact?
Note that this world is based on the Polynesia from 1800 B.C to 700 A.D so these people are expert sailors as well as have the technology of Imperial China except for the gunpowder. Also, note that there can be smaller ships too, the migratory ships are just the template.