One reason that hasn't been mentioned is that if you have no cities directly on the coast, then an enemy navy could demolish all your harbors and prevent you from ever rebuilding them, making you land-locked for the rest of the game.
Sure, but we haven't seen any reason why it would be a bad thing to be landlocked
housing bonus and appeal of coastal tiles could be a reason if you go for culture victory...
Furthermore sea-trading could indeed be a kicker for coastal cities - maybe they offer higher income combined with a harbor district which makes it worthwhile
As stated by others there might be possibilities to start navy earlier with a coastal city (no harbor district needed)
So I have a few hopes it might still be feasible to settle by the ocean...
The housing bonus is slight, and I don't see why we'd need the housing bonus if many of the workable tiles can't even feed themselves (1f1g water tiles, ew).
Haven't seen anything to suggest sea trading is different from land, and have in fact seen the opposite (both use the same unit)
If these things don't exist, then there's no value in a navy because no cities need to be coastal and there's no ocean trade to plunder.
But Appeal is a good one to point out. I had forgotten about it. Still though, you don't need to found your city on the coast for it, and water tiles don't have appeal (afaik). You'd still get more of those tiles, and be able to build on more of them, if you went slightly inland.
Maybe the water tile yields can be improved. You do get some fresh water from coast which is better then nothing which most land tiles provides.
Cities near the water can build a port which give a trade route. This mean a costal empire can have twice as many trade routes as a land based one.
Given that cities don't have an upkeep or penalty after founding you could maybe pack more cities in a single area is you build some of them on the coast. All districts get an adjacency bonus for being next to other districts so unlike older civ games civilization VI actually encourage you to build cities near each other so you can build alot of districts next to each other.
Haven't seen anything to improve the water tiles.
Good point about the potential for you to have double trade routes if you build both a harbor and a commercial district in your coastal cities, but again you don't need the center on the coast for that.
Ah, map overlap. That's a good one I hadn't considered. I think that's more like a "well I guess this is the best leftover spot", which works, but isn't really *fun*. I want a good reason to found my 3rd core city directly on the coast, as opposed to founding it by another mountain range.
There's the 3-tile-city minimal distance which may force you towards the coast (if overlap proves as worthwhile).
The harbor takes a full tile which can be more easily "blockaded" (pillaged) than a city so you may run into the problem of losing sea superiority (like air...).
There's tourism and appeal rates, trading routes being much more valuable and maybe port cities giving you a bonus to espionage (those gossiping sailors).
And I can't really think of many more examples for sea resources, so instead of making something up, sea tiles need to be improved by better tile yields. Just relax for when the game is released or we at least have more information
Already addressed these above, but good points.