The fishing boat tooltip says "Fish the waters for Food. Garrison a support or infantry unit inside to boost fishing rate." This patch implements that.
The question is how big and what kind of boost should it produce? Currently fishing boats have 1.8 f/s gather rate, 40f capacity, and 10 m/s walk speed. Of course their real gather rate depends on how far they have to travel. For example, fish 50m away from the dock requires a 100m round-trip, which takes 10s, plus 40/1.8 = 22s to gather the fish, so all told the real fishing rate in this case is 40/32 = 1.25 f/s. If the fish are twice as far away, then that's a 20s round-trip, so the real rate is 40/42 = 0.95 f/s. For comparison, berries are 1 f/s or 1.5 f/s with the "wicker baskets" technology (minus walking time) and full fields are 0.4 f/s (usually placed so there's no walking time). At the very least, garrisoning in a boat should add more than 0.4 f/s, otherwise we'd be better off farming with this unit. I think it should pay more than that, because there's micromanagement involved as well as a risk of dying to enemy ships. And fish, like berries, are a finite resource.
Here's a screenshot with one fishing boat slightly closer than 50m and another slightly farther than 100m (estimated using the round-trip times), just for reference.
If the bonus is an equal increase in rate and capacity, then it will take the same amount of time to gather the fish as before (since time = capacity/rate), so the real fishing rate increases by the same factor. I'm proposing a bonus of 80% per garrisoned unit, so an extra unit would effectively gather at a rate of 0.8 * 1.25 = 1 f/s for fish 50m away or 0.8 * 0.95 = 0.76 f/s for fish 100m away. I'm also proposing changing the fishing boat's garrison limit from one to five. Then a fully garrisoned boat would gather at a rate of 1 + 0.8*5 = 5 times their usual rate.
Why not just make more fishing boats, since they are 100% instead of 80%? They're cheap at 50w, but they take a relatively long 20s to make. Boats tend to run into each other, leading to inefficiency, and only three or four boats can gather from a fish at a time, although some can drop off resources while others gather. Currently there's nothing to do after the fish are exhausted, so eventually you have to delete your fishing boats.
I settled on a +80% gather rate and capacity and five max garrison, but let me know what you think. I also bumped the fishing boat's vision from 24 to 32 (to match women), because that was a bit ridiculous. The "fishing net" and "salting fish" techs both still work as you'd expect: +30% gather rate and +20f = +50% capacity.
I hope that this patch allows fishing to be a viable alternative to fields in the late early game or middle game, and that it makes water control slightly more important.