Campaign setting for any fantasy game - pulp adventure would be best. Remove any teleport/plane shift abilities: if you want to get somewhere, you sail there.
The Seven Seas are listed in reverse order, top to bottom. Rolling a d6 on the first few tells you where your characters are from.
The Seventh Sea: The Sea of Fire
Eternal darkness above, lit from below. Flame and molten metal, fierce red and brilliant orange.
Islands: Blood-black stone. Craggy, sharp here and smooth there, sooty and twisted.
The Locals: Skin of any colour seen in flames - deep red, pale yellow, muted indigo. Horns, the occasional tail. Passionate and devoted in all things.
Beasts: Few. Large, slow, rocky and terrible - probably invincible.
The Holy Orb: The Red Stone. Deep within a cave, it is given as the grand prize in a tournament of strength, combat, poetry and all other worthy skills. Consume it to gain the ability to grow in size and stature to match your massive, unconquerable spirit. Must be regurgitated and returned within the year as the prize for the next contest.
The Sage: A famed beauty, their words as silky and sharp as their blade. Learn a snicker-snack of the sword that can carve stone and steel and set the cutting edge alight.
Going Down: Nothing.
Going Up: A risky endeavour; the Black Sea is like a wall with the weight of an ocean. You will need a drill to pierce the heavens - local smiths are able to upgrade your ship into something resembling a helldozer, but such favours are serious things here, paid in mighty deeds.
The Sixth Sea: The Sea of Darkness; The Black Sea
Ink, murk. Any light is diffused and quickly swallowed, refracted on clouds of silt. Sound dies, breath fails.
Islands: Tunnels, spires, catacombs. Twisted reefs and mass graves. The derelicts of all upward seas.
The Locals: Whether they are shy, ethereal or simply do not exist is the subject of debate only among drunken sailors when all other topics have run dry. Who would want to stay here long enough to say hello?
Beasts: Shifting shapes in the dark, some small, some monstrously vast. Lights that blink alluringly then disappear. Muffled screams, plaintive pleas - these are warily dismissed as lures to a quiet doom.
The Holy Orb: The White Pearl, in the clutches of some monster. Swallow it and see - truly see - for the first time. Invisible becomes visible, illusions hold no sway, blinding light or crushing darkness are no match. All script is legible, though you must learn the grammar or code yourself, and the spirits of the dead that drift up through the seas towards the hereafter can be seen in great streams across the skies.
The Sage: Somewhere, in the carapace of a long-sunk cathedral, a white-haired fish sleeps in a bubble. Wake it, learn how to hear its words, and it will teach you how to find your way to land while lost on any sea.
Going Down: Falling through silt. Horrendously claustrophobic, but very doable.
Going Up: Keep going, even though nothing seems to come closer. Push through the despair, black out from exhaustion, and wake up on a shore in the Deep Blue.
The Fifth Sea: The Deep Blue Sea
Roiling and churning, yet changeable. Blue, yes, but green, black, grey. Goes through phases - brightly lit, with the Fourth Sea above reflecting its blue waters, then deep dark, with piercing lights from the Second Sea twinkling down.
Islands: Mud, lush with greenery. Occasional sand or mountains - sometimes ice.
The Locals: Various shades of brown, somewhat hairy. Nice enough but intensely tribal - ingratiating yourself into a community is simple, but be careful you don't find yourself allying to one side of a deeply knotted inter-generational feud. They are all storymongers, with myriad legends telling half-glimpsed truths of the other seas.
Beasts: So many! Bugs and fish, mostly, and the land is teeming with little hairy things like cats and pigs. Each island will often have some variant bird or special local creature.
The Holy Orb: The Round Stone. Old and dull grey, revered by each tribe for its supposed part in various folk tales. They are always stealing it from each other and placing it in ever more elaborate temples. Eat it and learn to speak and hear any language - the tongues of mortals, of the animals, of clouds and stars and ancestral ghosts.
The Sage: Skin like a nut-shell, beard like sea-foam. Cantankerous, giving little away. Follow him and study well, and you too can learn to fly.
Going Down: Hold your breath and dive under. Taking your ship along will require some upgrades, but the three mechanical parts you need are all made by different tribes that were once a single group.
Going Up: A flying machine?! Don't be preposterous... You sound like that one tribe who live in the jungle crater yonder and all worship birds. There's a really tall mountain somewhere, you can climb if you're desperate.
The Fourth Sea: The Pale Blue Sea; The Sea of Sky
Colourless, glassy and prismatic yet in motion. Far below and far above, distant seas are visible.
Islands: A single mountaintop, some fallen clouds - the rest of them, people made with clockwork and driftwood.
The Locals: Inoffensive pastel skin tones. Nomadic, free spirited, individualists.
Beasts: This is the home of dragons - stratospheric serpents that embody the weather. They, like the strange birds and flying fish that dwell here, never need to land.
The Holy Orb: The Bright Star. Intense white light, too bright to bear. Races across the sky each day like a rocket, then sinks down to extinguish itself in the Deep Blue below, where it disappears into vapour, only to be reborn again with the dawn. Eat it and you can walk on this sea, or any other - but if you do gobble it up, you'd better find the folk downstairs a new Sun.
The Sage: A young adventurer like yourselves, wandering in a flying greenhouseboat. She knows how to raise plants and animals from each sea in any of the others.
Going Down: Fall. Or, begin the long trek down the mountain.
Going Up: Make a deal with the dragons and they'll let you pass.
The Third Sea: The White Sea; The Sea of Cloud
Brilliant, pure white. Lit seemingly from everywhere. Has topography: pillars, chasms.
Islands: Architecturally beautiful, each wisp or mass of cloud like a different material.
The Locals: Very tall and beautiful, with white-feathered wings. Unless you can prove you've earned the right to be here, they'll assume you made your way up by some cruel trick of nature; something for them to pity, to remind them how well they have it.
Beasts: Winged horses and porcelain pigs, many-plumed songbirds and solid gold grasshoppers. Like your animals, but good.
The Holy Orb: The Cloud Heart. Deep in a bureaucratic labyrinth. Consume it, and this entire sea bends to your will - they'll beg you to keep it afloat. Will you be a tyrant or a benevolent god? In any case, nab some little cloud-balls while you're here that you and your friends can zip around on.
The Sage: It's a long and boring process, but you can learn how to manifest magic powers within your genitals to be passed on to your progeny. Now your kid can change their face or breathe fire or something.
Going Down: One of the many gaps - this is the smallest sea, and you can see through the transparent Third into the Fourth far below, down at the bottom of great chasms through which starlight shines from the Second above.
Going Up: Cloud rivers.
The Second Sea: The Astral Sea; The Sea of Stars
The vantablack void utterly spoiled with stars. Nebulae, galaxies, clouds of cosmic matter. The Moon.
Islands: Not here. Planets, perhaps, if you somehow navigate the endless expanse.
The Locals: Starthings, crystal chimes, tentacled ectomorphs, gas giants and red dwarves. Each on a ship or vessel of some wondrous design.
Beasts: There are star whales, sure, and alien monsters, but here it is intelligent life that rules.
The Holy Orb: The Smallest Planet. A little pink sphere with a yellow ring floating around its equator. Eat it, and you can become many - one naked clone of yourself born from light per star cycle, dying at day's end.
The Sage: A moon elf looking to go back home to one of the trillion other moons here. If you take them back, they can unlock their people's holy vaults, and give you the universe's greatest secret - a spell of teleportation.
Going Down: One of these directions is probably down.
Going Up: Not "up", per se - we've reached the end points of the universe's sense of relative positioning. But there are scholars and mystics on lower seas who know the way. Or just die - your soul will see the First Sea on its way out.
The First Sea: The Sea of Dreams
Every colour. Distance and form are forgotten. All seems close, embracing. A... "place"? Maybe.
Islands: I don't understand the question.
The Locals: All of you, in a way.
Beasts: Hmm... pass.
The Holy Orb: If you somehow do find it, it's probably God. (Or, the GM says nothing about it until your characters get there, then when you ask they just look at you, smile, and hand you a d20.)
The Sage: There are a few who dwell here, if "here" is anywhere. All know the secret of reincarnation.
Going Down: Wake up.
Going Up: ...?