My feeling is that if you are going to deliberately choose an alien and bizarre setting, you really can't go over the top with it. This is almost exactly the opposite of my take on normal fantasy settings, where I tend to think that going light on the wierdness and novelty pays big dividends in your ability to tap into real world mythic elements and thereby increase the emotional power of your setting.
But if you are going to make the setting an underground sea, then you should spend alot of world building time creating truly novel things because that's going to be alot of the fun of the campaign. You need to make your own 'myths' and lavishly paint your setting so that it can compete with things like Greek, Hindu, or whatever mythology.
Elements I would use in my underdark campaign -
Feared Aboleth slave/crime lords, ruling over far flung merchant empires.
Cosmopolitan intruige filled Darkcreeper cities where using a light is capital offence.
Goblin trading hubs, where races which hate each other mix, and crime is tolerated but not paying taxes gets you thrown in the arena for sport. Hobgoblin soldiery serve the ruling council of goblin priests, while Bugbear aristocracy and Ogre enforcerers roam the docks.
Free-spirited Drow merchant/pirates travel between ports in beautiful but bizarre ships of fungus and bone powered by spider silk sails. They follow 'the code', and democratically elect thier captains - but recognize that its all more like 'guidelines'.
Mindflayer slave trader/wizards plot to rule the surface, but first they have to gain control of the underdark.
Howling hordes of troglodyte barbarians threaten the fragile civilization.
Wildernesses of great fungal forests along the coasts conceal fearsome monsters of every sort.
Many different large caverns, each containing its own sea and surrounding communities. Each sea should be themed, strongly or subtly, and have its particular dangers. For example, one sea might have active volcanic islands in it, and so glows dimly red. This sea might be subject to clouds of sulferous vapor, and patches of boiling water from undersea eruptions. Another sea might have a ceiling covered with phospherecent fungal colonies and be densely inhabitted along its shores. On another, Duegar fortress ships keep the peace on thier sea with cannon and ram.
Cities ruled by ancient liches, powerful shade warriors, ghosts of a former civilization that don't realize that they are dead, derro alchemist philosophers, slumbering dragons, sentient golems, peaceful myconid librarians who catalog odors, and outcasts who have fled the surface world.
Ships and costumes should be bizzarre and alien. The backs of sea beasts, animated dead hulks, steam paddle ships, undead rowers, anything you can think of. Biological technology would be cool, like lights being specially bread phosporecent animals or funguses, and agriculture being based of harvesting mineral (or magic devouring) devouring algea/worms, and then building up to something that eats that which eats that which is edible. Let wierdness rule.
