Half-Life 2 Walkthrough - Chapter 4: Water Hazard - Destroying the Helicopter and Escaping
This section of the Half-Life 2 walkthrough covers the last series of battles against the armored cars and the helicopter at the end of chapter 4. This will also have the directions to open the last flood gate and end the chapter.
You'll enter this dockyard area at the end of your canal run. There are two armored cars positioned on the docks. The best tactic is to drive in circles around the ship in the center while shooting the cars until they explode. Once they're dead, park by the semi-crushed container at the end. Grab the energy from inside the container and then walk through the planks to the area under the dock. Then turn right and go to the intact container. Blow up the two explosive barrels in the container to blow off the doors. Take your airboat through the container and under the docks. Jump the ramp to get into another tunnel.
Killing the Helicopter
Time to finally kill that helicopter. Move forward and start shooting the explosive barrels to clear out the area so you don't accidentally ram them later on in the fight. The helicopter flies out in a minute. This fight isn't too hard in concept. Avoid the single mines and keep moving to avoid the gunfire. Just keep shooting the helicopter while this is going on. The only time to be really careful is when it sounds a panic alarm. This means that it will drop a ton of mines in the water and there is no real counter. Just shoot at the very bottom of the helicopter to blow up the mines as it passes over you. This should create a safe zone and limit your damage, as long as you don't move. After a pass or two, you should do enough damage to blow it up.
Third Flood Gate
This is the last flood gate. Take a moment to look around. Go to the center of the spot where all the explosive barrels were. You need to take the ramp on the right to fly through the gate, but it's closed right now. Go to the left side and climb out and up to turn another valve and open the big gate. Drive up to the docks.
It's possible to take the airboat up to the building, but it isn't really much of a benefit. Just climb up and move toward the red building. Shoot the two soldiers that come out and then throw a few grenades through the door to hurt the three inside. Kill any survivors. This is an ammo storehouse, so stock up now. Use both chargers, pick up all the ammo, and maximize everything. Step outside and move toward the gates along the side. Cross the bridge. You should be able to spot the area since the G-man will be there for a minute. Walk out to the top of the flood gates. Kill the soldier who might be up here and go through the gate. Pull the one switch to open the gate and then backtrack to your boat. Take the ramp to drop down into the water far below.
This officially ends the chapter.