I'm pretty sure canals give bonus gold to trade routes passing through them, at least that's what it says in the civilopedia and the Civ Wiki. I haven't checked the numbers myself, I'll probably do that in my next game. The tricky part would be figure out what exactly the bonus is, it will probably be lumped in with the category "from other bonuses".
Currently what canals do is they add to the "route efficiency score" modifier that is the same bucket that water tiles and railroads and mountain tunnels add to - it caps at 100% of the
base gold value of the int'l route. (Things like zimbabwe and various cards and merchant effects are added separately.) The Panama canal tile itself gives you +10

for owning it, but otherwise is exactly like a regular canal as far as I know. (someone correct me if i'm wrong)
So if the route is already almost entirely on water, the canal adds 0

to your trade route.
Also makes the coastal IZ for Venetian Arsenal more worthwhile. It's often hard to find a high adjacency spot for that.
On these new maps like small continent and seven seas where navy is more useful, venetian is much more useful. But I have always been struggling with this too! I either put my IZ Inland or if it's on the coast usually there's a harbor of something there instead. I hadn't even thought of this!
Unless I'm reading this wrong, IZs didn't really get the massive boost that was thought, only a marginal boost that barely impacted gameplay as it turned out. This makes me sad.
It depends on your playstyle. Just using mines and lumbermills now is no longer going to cut it; however, it has become much easier to get +5 and above than it was before. But you have to adjust to using more of the green districts - AQ, dam, canal. If you do, then production has gone up significantly,
especially in otherwise low productivity cities without a lot of hilly areas. Some civs (japan, dutch, germany) have really benefitted. I mean I could repost examples from other threads but those guys can get boosts like +8, +10, (Germany mainly) +15. Anyone else can still see 6-8 in a fair number of cities just through planning and a little map luck. This is way better than before, but mostly only if you use craftsmen card.
I built more IZs than I usually do but even though there's more types of adjacency, I feel like it's harder to get good adjacency where I need to place the IZ with regional effects in mind, because now it require two mines so you end up building the IZs near stronger boosts, like strategic resources, quarries and Aqueducts/Dams. I also built more aqueducts than I usually do.
I get the sense that the IZ is now intended to be something you build more of than you need just for regional coverage. Between being able to get higher adjacency in more cities than before, the workshop giving +3, and coal plants, it just seems like building 4 IZs when you could have covered everything with 2 isn't a high crime against Sid Meier. Plus, more production is always fun!