Sorry if there are typos and its a bit long, am just unloading and going as am a bit busy
The main problem with coastal comparison is if you do the math on paper it can seem good or even great. But if you play a lot of coastal games (I mean like 30+) then you you just start saying it sucks without even thinking about why, you just know you suffer in so many starts.
For a start half your territory is water and some (sometimes quite a lot) is ocean and unusable until seasteads. The coastal water is 1 food 1 gold, just pointless. But of you do get some amber or turtles you get rather excited because it is not that often you do and the rest suck a lot. I do agree fish and crab seem great. I mean fish is a bit of gold better than swamp (woooo) while crab gets you 3 gold (3 is lots right?) Reefs are a nice addition until they appear at a river mouth and the mothers often do because they just stopped me putting a harbor there (why can I not destroy a reef?) So what is often forgotten is I have 4 desert tiles, these reduce my viable land by twice as much as any landlubber city. Hils appear at coasts but an awful lot less than next to mountain chains. You get happy when you have 3 hill tiles within 2 rings on the coast while it is standard for inland cities.
So the theme so far with the coast, just looking at the water is gold. So what value is gold? Why does everyone (including Firaxis) make the assumption that 2 gold = 1 production? Buying units is 4 times the production, buying buildings is 4x the production and yes, they just nerfed upgrade gold and therefore nerfed gold civs like coastal civs. Does gold get me science or culture, no. So it is inherently worse when you are getting it in quantities greater than what is needed for maintenance and upgrade. 1:3 to me is still too low but lets used it. So that means a 1 food 1 gold tile is a lot worse than plains and we know how crap plains are. A 1 food 3 gold crab is good as ... you got it, plains. And the way I look at it a commercial CS should be giving me at least 6 gold but nope, I get less of an inherently worse reward.
So lets get something clear with settling. For a start half of my games start without me on the coast so I have to move if I can even see the coast. the majority of starts are not on coastal river and the housing is just appalling then. I ask for more housing because this area is just terrible. Often forced to build granaries which means I have to beeline them and build them with poor production (not enough gold early) and that means I am not beelining harbours or Commercial districts.
Mountains are on coasts yes... and sometimes you can get luck and get more than 1 but +3 adjacency mountains are very rare and typically are completely useless for adjacency of other districts... but sometimes they are at the mouth of a river like a luxury is, stopping you build there... and with coastal cities it is not about I can build at option b because it is 90% as good. If you cannot settle on a specific tile the difference can be large. And harbour triangles (I claim copyright) are a false economy. You are mounting a large gold approach at the cost of a trade route and with no science or culture yet produced. So everyone shouted... "Free Inquiry!"
I got excited when I saw it being introduced and did threads on its possibilities... and this I think is the issue with the OP, looking at the paper value of things. Upon playing free inquiry you realise is is a honey trap. You can only use it for 2 ages and yes you get ahead in science but come out the other end when people are getting huge science bonuses now from many cards, CS and abilities so their cities are on 15-30 science each... and suddenly you find yourself on a few fisherman occasionally thinking of a better way to mend their nets. If that was not bad enough, free inquiry does require a golden age and you can often forget that on immortal +... a golden age is a lot harder to get if you are concentrating on granaries and beelining harbours. In my experience on Emperor about 1 game in 4 I can get a golden if I am trying to get 3 cities out with habours and commercial hubs before classical. The other big problem with free inquiry is it last 2 eras. That and a lack of science after make it a honey trap.
Fisheries have only one use, and that is a large food source so you can grow above your limited housing by force. And these fishing nets all disappear when a hurricane comes. Hurricanes truly rip a coastal city back to the dark ages. Hurricane are HUGE, why in gods name does a CAT4 cover the same area as a CAT5 and do similar damage? and to add to it I have been hit by sandstorms and blizzards in coastal cities. The thing I have to worry about least is droughts, the thing I really miss on coasts is volcanoes and fissures, they are a huge free gift.
One of the biggest problems is science, getting it is the lesser problem. The tech tree is just crap for coastal cities, I beeline harbours which is not fast and I still have slingers and warriors at T40 when chariots and archers are on the move with AI's. Playing a coastal game is a gamble. The commercial hub triangle is also an issue here and you will find the best way to play is in fact a campus, harbour triangle because you need the science to survive and now they nerfed cartography coastal cities got it right up the buttress, once again. I need granaries as well, and I want to get to shipbuilding as soon as possible for quads because my galleys do not cut it (unless Phoenicia or Norway) and I want to send my settlers to sea. I want to get to commercial hubs, granaries, archery, bronze working (I am England), celestial navigation and shipbuilding.... just what do I research first? Placing harbors at shipbuilding is about the most sensible solution and I played maybe 40 games with this change in place and it makes the game more playable but in no way OP. The 2 tree leaf to harbors is a JOKE. People were building harbors before libraries.
To be fair we must look at this in a balanced way. Lighthouses do now improve our coastal areas to 2 food so they are now 2 food and 1 gold, a touch more than grassland but they pump up resources which is nice. The +2 adjacency for harbours can be pushed to 5-7 if you are lucky with adjacency and IF you are lucky in that regard then the double adjacency card will put you at say 12 which is the same as +4 production.... +4 production IF using a card and having got your city just right. It is not as great as you think when you convert it to production... but the one saving grace is shipyards. Shipyards with then give you +12 production or +18 with Reyna but I must stress this is in 1 city with Reyna (sacrificing Magnus/Pingala/Amani benefits) and the lucky one or two other cities are normally around +10 production. A seaport can give great gold on top and you can get some really fat golden coastal cities later but when you start 3:1 the gold it is not that great for the loss of production.
Double trade did help but I get no road benefit from that and it is hard to protect at sea.
Bottom line is coastal cities come out much less than par with inland cities and more importantly you have 0 flexibility with them. You have to build in a specific way for a golden result that can also to a degree be mimicked by an inland city if it chooses to go that route. With coastal cities you sort of have to play the naval game which is just dire. yes your frigates are OP and you can rule the waves easily. That is half the problem, it is just snooze time and now with nitre restrictions is just more effort for an dull approach.
You play a civ with 1-2 coastal cities and you do not really notice but play a vanilla start civ like England where you need the coast for a lot of cities and its more of a struggle than you can see on paper unless you really think hard.
If your city makes 60GPT you may think that’s really good, but do you think that if your city is on +20 production? I would argue its closer to +15 production.
Oceans are unimprovable, but they have better yields than mountains.
You cannot put a civilian on ocean so it has no value until seasteads. The adjacency value of mountains people always talk about. They are both “yields’ but I know which I would prefer.