Where You Build Your Gun. So build high, and let those rockets rain down. You can also build on pretty much any flat surface, if there's room.
Most maps have some water tunnel.
Just as an infiltrator is about to drown, then they have to swim through a river of bullets. And make sure you come up for air just before you start to build, else the first body next your gun will be your own, from drowning. Stand on the edge, so the drop-off is on your right side. Build, then turn the gun twice. Voila, it's facing out.