Well, the map I'm working on is entitled Submarine.
The human and alien bases are on separate ships, with a considerable amount of water between them. There is a low-lying cloud layer over the entire map, which contains a trigger_hurt to keep jettards from getting too crazy over the ocean, and acid fog surrounding the area's boundaries to keep grangers from going for a new longest-swim record.
Linking the two ships, there are underwater tunnels, overwater dinghies with buttons on them to activate a func_train entity, and a teleporter on the main deck of each ship. The teleporter makes it necessary for bases to be moved, otherwise you'd have S3 stalemates such as those in ATCS and other similar maps with no alternative base options.
Partially submerged in the middle of the ocean is a submarine. No tunnels link this submarine to anything; the only method of entry is swimming, underwater submarine units capable of holding approximately 4 humans at a time on a func_train route, and overwater dinghies.
Inside the submarine, there are various rooms corresponding to those of such; missile room, command, living, etc. This is where just about all of your fighting takes place.
As for alternate bases, you can move anywhere within the starting ships, into the underwater minisub docks, or into the hellhole of a submarine itself; if you're gutsy, moving to that last one.
So, I can't really do much structural brushes to reduce vis to any particular area, except the ships, I guess. Should I do hulls of the ship and submarine in structural brushes, and the rest in detail?