I didn't get off to a good start. All my ammo got used up killing the teleporting imps in the first hallway. I had to punch the last few to death. Obviously, this ammoless state did me no good stepping into the first main room. Got cornered and shredded a number of times before finding enough ammo for half the enemies. Took care of the other half with infighting. Might've gone better if I didn't kill the imps, but you have to account for people killing whatever enemies they can, even if you don't consider them an immediate threat because they're caged in.
Things got harder after that. After checking both the main doors leading out of the room, I took the lower route to the Metal2 area with the water pit. Ran out of ammo again, tried punching past cacodemons and the mancubus. Made it to side room, where revenant rockets fired from way across the main room while I was in view of the window came around to kill me when I backtracked across the room to the door. After getting blindsided by a stray revenant rocket for the 5th time, I just threw on God mode and kept playing.
And it was SO worth it. First off, I love the design. You kept a consistent underwater-base feel throughout an incredibly large variety of scenes, and you managed to use an extensive amount of textures in a well-coordinated way so that the settings stayed fresh without looking like you just jammed unrelated maps together. Second, the layout is great. This is one of the most non-linear maps I've played, and the wide variety of paths to choose ensures I'll probably never play this map the same way even if I play it 10 more times. Third, IT'S HUGE! I think the last time I played a single map this big was Deus Vult. I took a little over 3 hours to beat it, and it was 3 hours incredibly well spent. Also, props for using music that didn't get tiring after listening to it for so long.
I think the main thing you need to consider when designing such a non-linear map with so many possible routes is that few people will play it the same way. I'm sure you have your own way of getting through the map without much difficulty, but I constantly found myself punching pain elementals, barons, revenants and other tougher monsters during the first parts of the map (even with God mode on to ensure I wasn't wasting ammo by dodging and shooting simultaneously). Maybe you're the kind of player who picks one route and runs it to its end, but I'm the kind who sees two or more routes and alternates back and forth between them. You need to have every possible way of playing this map covered. Maybe you gave me enough ammo for the next scene of one route, but I took it and ran to the next scene of another route. I think a few more boxes of bullets and shells throughout the first third of the map would cover most of the play options you may not have considered.
Finally, it's not always clear what the switches do. Most operate something in their immediate vicinity, but others open something halfway across the map. This would be a great place for ZDoom's security-cam feature, where you flip a switch and temporarily get a view of what it's doing in another part of the map. But if you want to keep it open to a larger amount of ports, you could use signs like "exit closed/open" or display monitors with map screenshots that change when the switch is pressed.
Anyway, I hope that didn't sound too negative overall. This really is one of the best maps I've played, and I'm sure I'll play it a number of times again. It just needs a bit of tweaking - and that'll take it a long way.