I'd absolutely love to have an underwater map. I posted a while ago about an underwater map on MonCal, where the mon calamari over through the imperial forces on their planet. I think that an actual battle (in star wars...) took place this way. There where At At Simmwers:

And Seatroopers:
And these guys from the clone war mini series:

I think before some said it was impossible to have an under water base in bf2. But now with 2142, I think the devs mentioned they where looking foward to using some pieces of the titan coding (thingy) to make echo base. Could this same coding go towards making an under water mon cal base? It could be completely enclosed (as it should it be), and the way people could exit it could be a system similar to the jump pod launcher on the titans. So, there could be an airlock section of the base, with little air lock pods. You'd get in em like the pod launcher, you'd click, and you'd come out as a quasi infantry/vehicle that can manuever around under water. There could be a similar airlock bay that lets the mon cals come out on those fish mounts.
The map might have several corral ridges scattered that are flags, and need to be captured or held. Perhaps there could be some defensive turrets that could be spawned into at these flags, in a similar fashion to the defense turrets that floated around in space in GC.
The map could be won by either side by destroying the opponents base. The imperial base might either be an acclamator landing ship, like it was in the clone wars series, or maybe a smaller landing ship floating just above the water. There could be a small floating 'dock' area in position beneath the floating ship, where the moncals could reach, get out of their under water gear vehicles, and assualt as normal infantry, pushing up from the dock into the ship and destroying it like a titan.
Alternatevly, the imps could assualt the mon cal base. There might be a capturable 'flag' floating just outside of the mon cal base. This flag spawns anyone who uses it inside the moncal base. Perhaps the flag could be right outside a large air lock bay, so it would make sense. So if they can capture the flag from the outside, they can assualt the base from the inside.
And thats about it. I love the idea of underwater, in case I haven't mentioned it before. Is it possible?

Futurama: You Can't Prove it Won't Happen