Fishing as a food source

I thought it too.
Just adding some outposts which give foods (as "seafoods" items?) will be a simple and nice feature.

Now that we'll have ship pathfinding in U4, I think this becomes a lot more feasible.
I read the objections above, but I think it can be balanced well:
If it acts like a groundwater source, then it means total fish you can get is capped, so you're still incentiviced to build other food sources.
It could also provide incentive to straighten out a particular shoreline that you otherwise wouldn't bother with, if it gets you closer to fishing grounds. I imagine that ends up reducing the diesel per fish and increasing the fish per month per trawler because of the reduced travel time of the trawler.
Also, map makers could use it to incentivice keeping some natural shoreline, if dumping over the fishing grounds effectively eliminates it as a source of food!

I was also thinking of this, along with using meat trimmings as "bait" that functions similarly to fertilizer for farms. You could also use this for fish farms eventually, further down the tech tree.

I like this quite a lot. Fishing dock to start with in the early game, combine this with a mid-game ability to send out a cargo ship as a fishing trawler/factory ship and comes back to drop off at a cargo dock as normal.
Adds an early source of protein that doesn't really run out, enabling you to cut back on the number of farms you need.
I would also add an ability to stock any manmade lakes with fish, so that you have the ability to source fish from inland.

I would like to vote against or at least request caution in how this is implemented. What makes CoI a fun challenging game is managing logistics and limited flat land. Right now food comes from large flat areas. If I can drop a fishing dock next to my settlement, I've just trivialized the logistics and land management of food supply. So if this is added, please at least make it need some sort of meaningful footprint on land so starvation prevention isn't trivialized to "just spam fishing docks on the coast".

Previous poster said it would cut back on the number of farms needed, and I think that's exactly the problem. If people want fewer farms they can lower food demand in the difficulty settings. If magic ocean food gets added, those of us who like the challenge are left in a bad spot with the way unity and food interact in CoI.

Regarding the idea that it's capped like groundwater, I don't think that solves it. Water has other (more costly) sources, needs to be transported, and has multiple sinks. Water has tradeoffs and decisions to make. Sure harvesting all the groundwater you can is mostly a no-brainer (as long as pipe maintenance isn't outrageous), but whether and how much you go beyond that is a meaningful decision, as is where you allocate your water beyond the mandatory consumers (settlement, farms, copper electrolysis either directly or via acid, oil distillation 1 and 2).

Fish you would harvest up to the cap, and send all of it to your settlement. There's no interesting decisions or gameplay there, it's just a chore. The only potential decision to make would be whether you limit the size of your settlement so you can satisfy the fish demand for more unity, or to outgrow the ability to supply fish for a higher population. Even that isn't a hard decision though.

I don't think the concern is warranted here. Most players typically try to provide their settlements with all food products to produce as much unity as possible, and this would basically just add fish to the protein food group, 3/4 of which is currently chicken based.

Even without all the byzantine restrictions you propose spamming fishing docks on the coast would, at best, just partially replace spamming chicken farms. It's not going to replace all the things you get from regular farms. Neither is particularly interesting.