Level -1: Shipwrecked. A self-contained world from which egress is difficult and limited, lacking a few of the dangers of the Constant proper but replacing them with tropical horrors of equal or greater lethality. Its edges wrap around to each other through transcendental fog. Even reaching the world of DST from here is a challenge.
Level 0: The recently-made-passable ocean of Return of Them, and the occasional saltwater lake enclosed inland. It has small waves, it has seabirds, it has wind but not to excess, it has kelp, and it has outcroppings of rock which cannot be mined and are fit only to smash holes into poorly steered boats. It can be used to access the opposite end of the island, or the fallen moon, or further places. Even on the outskirts of the true ocean, the shadows cannot seem to reach you in any tangible sense.
Level 1: The ocean across the border of the world. Darker in colour than the familiar ocean, whose waves reach perhaps a few feet into it and then break on an unseen barrier. The surface is utterly still apart from the immediate wake of whatever boats may sail upon it. No seabirds land here. The wind can still be harnessed but does not disturb the water’s surface. Dropped items fall through the seemingly solid deck of your boat to the waters below.
Level 2: Things do not stay in one piece from instant to instant. The parts of your own body are constantly flickering in and out of existence and clipping through each other, as are the parts of your boat. Your hair is visible through your flickering hat. Your scalp is visible through your flickering hair. Insides can be seen through outsides. The world can no longer be mapped. The wind is still here. The sun rises and sets. The shadows can see you, but they cannot catch you. The hounds, however, sometimes can.
Level 3: You and your boat become invisible except from a few specific angles, and cannot even be seen by a disturbance in the water. Sound still works reliably, as does nonvisual perception, and may be your only way of changing course to return to the world.
Level 4: You can no longer be seen at all, nor can your boat.
Recent innovations in boating technology have made it possible to pass from the outer reaches of level 1 to level 3 without ever seeing the telltale space-rending effects of level 2, but its existence - if that is the correct word - has already been documented. Whether the current state of affairs is a shortcut, a suppressant, or level 2 being subsumed into its neighbours has yet to be determined.
There may be further levels, but I haven’t visited them, and I know only one person who has.