Water Hazard is the 4th chapter in.
Starting at roughly the moment the player acquires the airboat, Water Hazard continues the story of Gordon escaping from City 17 and heading for Black Mesa East. In his way there are many Combine run facilities and some rebel strongholds, as the canals are being used as an escape route for many besides Gordon. The chapter ends at a large dam, where a nearby waterworks facility houses Black Mesa East.
In Water Hazard there are both natural rivers and man-made canals. When creating natural scenery, the materials used should reflect that, with natural barriers instead of the concrete used in the canals, grass, mud, sand and clear water with occasional pits of polluted water. Like in Route Kanal, most of the man-made structures should look heavily used and later abandoned, or at least very dirty. The canals are littered with large amounts of trash and debris, much of it metal or rock. Abandoned ships lie at the dry bottom of the once-trafficked river.
Buildings should only be of maintenance or industrial type, with most or all under Combine control. Especially dams and water locks should be operated by the Combine, and warehouses should house weaponry, ammunition and vehicles for them. Civilian housing should only be visible if the designer really wants to emphasize a feeling of being surrounded by a city. Furthermore, in the canal maps, such buildings are best kept in the 3D Skybox.
This is the portion of the game where the airboat is used, so player accessible spaces can be wider, longer, and incorporate more interesting traps and tricks (such as jumps). There can also be all kinds of tunnels and indoor canals, but they should be rather linear. Steering the airboat in tight spaces can often be frustrating. Also, beware of making jumps that require the absolute maximum speed of the airboat, as accelerating to that speed could potentially be impossible for the player.
Water Hazard starts in early afternoon and ends late in the evening. Other times of day can also be used for additional visual impact.
The Citadel is visible in the 3D skybox at first, giving the player some bearings on how far from City 17 they are, and later disappears from the horizon as the player moves farther away from the city.
Sounds should consist of water, quiet wildlife and loud pursuit sounds. The canal levels have a feeling of escape that is aided by the sound of pursuing helicopters, gunfire, and other hostile noises. Inside buildings soundscapes should be chosen accordingly, with machinery and air venting sounds in particularly frequent use.
As the Combine continues its hunt for Gordon, he is faced by an increasing number of Metropolice. They mostly try to shoot at the moving airboat from further away with their own weapons or APC rocket launchers. There is also an attempt to block Freeman's way by collapsing a smokestack in his way. Similar scenes are an option for both visual effect and directing the player on his path.
As always in watery surroundings, barnacles are found in tight spaces and under bridges. They can pluck the player out of the airboat, so give thought to their positioning, as their purpose should be kept at directing or diverting, not stopping the player.
Helicopters can be used as a kind of a boss enemy, or alternatively for visual effect via not giving it instructions to shoot at the player.
In Half-Life 2, The Hunter-Chopper was used as a pursuit vehicle, chasing Gordon and dropping floating mines in his path throughout the level.
Dropships are seen flying around dropping Metropolice in Gordon's path.
The red barn houses a former rebel stronghold, station 12. It was headcrab shelled shortly before Gordon's arrival.
"A Red Letter Day"
Black Mesa East
"We Don't Go To Ravenholm..."