Half-Life 2 Walkthrough - Chapter 4: Water Hazard - Gordon Get Your Gun
You finally get some firepower for your airboat in the fourth chapter. This section of the Half-Life 2 walkthrough covers the prelude to the final set of confrontations and it covers the last canal run. It's a short section, but you might need a little help to get to the end with full health.
Getting the Gun
The helicopter is still here, so be quick and turn to the left to get to cover inside the tunnel. You will finally make it to a rebel base. They'll raise the bridge and have you dock. Follow the rebel inside for a short explanation that would be meaningful if the path wasn't linear. Just wait until he says it's time to leave. Follow him back to the dock and wait for the vortigaunt to finish with the gun. Jump into the boat and wait for the gate to open.
Feel free to practice with the gun by shooting the barnacles and the junk blocking your pathway. The ammo recharges fairly quickly, so don't worry. Get out of the tunnel and go through the gate on the right. Six combine will try to ambush you, but you have a good gun now. When the gates close, spin to the left and shoot the three soldiers on the wall to the left. Then back up and get onto the fence that bridges up to the ledge on the right. Kill the three other soldiers and get past the gate.
Fourth Canal Run
Another armored car is waiting and will start shooting rockets ahead. You can blow it up, but it's a little pointless. It's probably better to just shoot the rockets to stay safe until you get past it. Go to the tunnel on the right and punch through all the combine waiting on the ledges. It's a piece of cake with your gun. Follow it outside to see another armored car. Shoot the rockets and get out to the end of the tunnel. You'll see a dropship. If you want, you can blow it up and make it drop the armored car into the canal.
Go ahead through and watch out for more barnacles. Then you'll come to a fork in the canal. Just ignore the Combine position to the right. The supplies aren't worth the fight. Go to the left and go up the ramp into the tunnel. Kill the barnacle and run over the zombies to get through the little tunnel.
There's another blockade up ahead. They'll set the river on fire but it shouldn't matter at this point. Rush through the flames or swirve to the right. Then go up the ramp and dodge all of the explosive barrels. Use your guns to blow them up at a safe distance. Then powerslide to the left and swing around to the raised section. Run through the combine and turn to the right. Go past the ships into the big area ahead and get ready for a lot of fighting.