CA50 is from volume 5, yes.

There are trees in regular DD3 style too. Just click the :DD2SKIRMISH: button. CA50 also added additional trees and vegations for this style.

Here's the basement level below the underwater temple. Also flooded obviously, so I used marine dungeons for that too.

I took some liberties with this one too, for example I used lots more sea plants that would naturally grow in a place with basically no light at all. I've found that when using this style, if you want it to actually feel underwater, you may need to overdo some things that might not be completely natural.

Had a bit of fun with this place. Considering the players had to use spells to breath and move efficiently underwater, they were on a timer. Unfortunately, they are quite high level by now, so with the spells active, they were more or less able to ignore the environment. But the timer meant they couldn't just decide to rest to heal up and recover spells, and several spell slots had already been used up for the underwater support spells. Also meant that Dispel Magic suddenly became the most deadly spell in the book.

(Hi Res)

Well, it is easy to know what's up when you are at the bottom.

It doesn't matter at all. Units in CC3+ are meant to represent real world sizes, so you make the map in 240 x 180 if it is 240 by 180 miles (or feet/meters/kilometers depending on map type) or 2400 x 1800 if it is 2400x1800 miles.

For an overland map, symbol scale will vary, as you pointed out yourself, and many drawing tools have line width expressed as fraction of map size, meaning they will vary too.

For dungeon/city maps, the larger map will give you a larger area to work with, since symbol scale don't vary with map size.