I expected canals to be the equivalent of railroads. I'm very surprised to see that they take up the whole tile as a district.
I expect they'll be treated like neighbourhoods, i.e. not count against the city district limit.
I'll be disappointed if they do anything other than allow ships to travel on them. I mean, that's all real canals do. They should probably come with a maintenance cost, too, but I could see dropping that in light of the up front production costs. The Panama Canal Wonder would obviously provide benefits. But a normal canal? Providing any positives other than the shortened movement for ships is likely to seem cheesy.