I've noticed that a tundra square with fresh water allows for farming and even a cottage, when tundras are ordinarily good for nothing with their 1 food. I'm not sure I've seen desert squares with fresh water before, but I don't recall fresh water doing anything for ice squares.

Has anyone else noticed how fresh water affects terrain workability?

Yeah, tundra tiles are weird. Besides the irrigation/cottage thing, it appears that they don't "carry" irrigation like other tiles do post-Civil Service. That is, only tundra tiles immediately adjacent to water can ever be improved.

As far as I've seen, it's only tundra that has any kind of special rules though. Everything else works as expected.

Fresh water allows irrigation pre-civil service, and gives a city located on a fresh water tile +2 health (IIRC). It's the only way to farm tundra, even after civil service, but you can NEVER improve desert (though you CAN get the +2 health from settling on fresh water desert).

This isn't exactly a fresh water issue, but irrigated resource squares that require a farm improvement to harvest get +1 food (e.g. a corn square that connects to fresh water via irrigation, after civil service, gets +1 food).

This also isn't a fresh water issue, but a grasslands hills allows cottage while plains hills doesn't. I guess it's the same theory as the tundra -- the cottage "needs" more water to be built?