The overarching map is a collection of islands of various size as well as some stuff out in the sea as well. Islands are scattered around randomly on the 2D grid, whereas oceanic POIs happen on tiles without islands around them -- they do generate in the map but don't actually spawn unless that condition is met, so there are stretches of empty sea.
There's a wind currents system in vertical blocks that go east or west. You can't move against the wind, but you can tack, or just go down or up to more favorable winds.
If the wind runs east, then going east is full speed, ne or se -1/4 speed, n or s 1/2 speed and nw or sw 1/4 speed. Can't go west, as mentioned. Sailing a tile following the wind uses 4 hours by default, but can be upgraded.
Time affects time of day, the weather (which can be *bad*) and your hunger and thirst. Food and water is relatively plentiful on the ocean, but if you die at sea there will be various Obake debuffs to ship rooms, which can be annoying to deal with.
Dying on an island, meanwhile, locks away all items you picked up on it behind an Obake.
Your ship has a Figurehead. It can be changed to various things that you find in islands via your equipped Magnifying Glass (different types do different things), and using it on some amount of the object in question. Magnifying another object while in the course of doing one will reset the counter (with a confirm box and some info on how many it will take, and the stats it produces).
Anyway, figureheads have various minor magic-like effects but their main utility is in combating Obake.
Obake demand some kind of sacrifice of item types in order to remove them. This can be quite annoying, but their effects are probably worse (particularly locking away items). You can only work with one Obake at a time and can also only activate a new one once per day, so they're not a great extraneous inventory solution.
Islands have some fixed entrance point. There are larger blocks of terrain connected together in sane ways. Crossing from one terrain block to another takes quite a lot of time.
Terrain types also have subtypes that go down fractally in several layers, reducing the time needed at each point. At the bottom layer, it's some kind of 2D structure (a moving line, a grid, several lines, etc) with some fixed entry point and possibly other exit points. These can be pretty mazelike, especially since the game is text-based.
Bottom levels are where all the resources are, as well as the entrance into POIs.
The amount of layers, types of bottom layers, basic terrain, etc are all procedural depending on the island. Some islands end up being quite large and death can be pretty likely if you end up deep into a maze and don't remember how to get back to your ship.
