Warhawk (SST about where you are in HL2):
the part where I'm in the airboat-thing and I shot down a PITA bomb-dropping helicopter in a relatively open area. I had to get out and turn a handle on a platform to open a gate to proceed. Am I near a dam or something? I just took out the 2 rocket-launching vehicles and drove under the dock to jump into a tunnel to get to this location.
While you're there, here's something you can do that's a fun little extra in the game (i.e., not critical to the plot).
Get full health after you go through the gate (the one you've already raised). Do what you gotta do in there first if you want, but before you leave that side of the gate, look along the border of the valley for 2 sewer grates in the wall. One of them will be partially open. Stack some stuff up and run inside. You'll lose a lot of health from the sewage, but don't worry about it, just keep going until you've found the secret area.