I don't think it's a problem of yields. It is a problem of housing, especially for coastal bias civs. And a problem of automatic tile gains.
Coastal spawns should either have a mountain adjacent or 1 hex away OR adjacency to lake or a river. I don't think most generated maps would have issues granting you one of these.
Also, culture tile gains should either beeline sea resources or go inland for production tiles. I find current coastal culture gains to be simply a bug, because they make no sense whatsoever.
To be fair, yes, its easier but not *that* easy to find good inland city spots either. Most spots are either good for resources, food or production.
Good points about 'tile gains', which are frequently counter to any logic or historical counterparts.
(Fresh) Water access is already 'programmed' into city growth, and should certainly apply to coastal cities as well, with one caveat: when small, or average in the ancient/classical eras, many cities had fresh water sources that don't show up on the map at Civ VI scale. For instance, Athens was built around the Acropolis, an easily-defended spike of rock in the middle of a broad flat arable plain (in fact, the largest such plain in all of Greece). But what made it a perfect site for a small city was that the Acropolis also had fresh water on top. Not enough for millions or even tens of thousands, but enough for Athens to have been founded as a city long before Greeks got to Greece (Athens is not a Greek word, the site and the city were settled by the pre-Greek 'Pelasgians').
We might, therefore, argue that (rarely) a tile normally lacking water access might be programmed with water access to represent springs, natural cisterns or other 'micro' water sources. Such a source might only provide 'water' for, say, the first 2 - 3 population points: not enough to get you a second District, but enough to have a 'strategic' site and requiring a lot more work on your part to build it into a 'major' city - as Athens eventually piped water in from the nearest mountains/hills to the north and northwest.
I would argue that Hydraulic Engineering for Water Supply is a type of 'Irrigation' that the game ignores, given that many ancient and classical Civs spent a great deal of time and effort ensuring water in areas that didn't start out with it: "aqueducts" are only one type of water supply that included springs, wells, cisterns, artificial lakes (barays), tunnels into and through hills and mountains, covered water courses (Afghanistan's qanat system that stretched for miles to 'irrigate' both fields and towns). Some of these could legitimately be UAs or UIs or UUs, but others were generally used by anyone who developed the digging, stone working, surveying technologies required.