The few man-made structures along the coast are houses, shacks, lighthouses, and towards the end Combine bunkers and forts. Don't forget to include thumpers where appropriate, if the Combine have felt it necessary to repel the ubiquitous antlion hordes.
Houses should be simple. At most, 4 rooms for a single-floor house is advisable. They should be wooden and plaster, with enough props so they appear usable. Some damage, such as missing roof or floor sections, is expected. Fireplaces are very common.
Bunkers should be fairly simple, using concrete materials and Combine props. They should be along coastal sections. They should be kept in areas seeming near Nova Prospekt. Combine energy fences should be rarely found in the bunkers (and never on the coast alone), and should always be passable or possible to disable.
The majority of surfaces should be natural textures such as sands, grasses, and rocks. The "cleaner" water materials work best here. Vertical natural faces should be rocky, while flat surfaces close to the water take the sand. Grass should be kept away from the actual coastline. Because of how antlions spawn in Half-Life 2, the player may have learned to be wary of sand. Use this to your advantage. Provide them with rocks or debris to move across, which will help to direct them toward the intended path. Since the Combine's arrival sea levels have fallen, so try to design your landforms and older buildings for an environment with a much higher tide.
As noted, Thumpers are often used along the coast, so consider placing these within large sand areas. They also create an ambiance in a level that suggests the presence of antlions. The ground shaking created by thumpers scares away and confuses antlions, who refuse to move within 1000 units of it. This makes thumpers a useful tool for creating rest points for players to reach. However, remember to have the thumper present for a reason, as the Combine would. Place them near buildings and strategically or otherwise important locations.
A coastal level can be either day or night, depending on the mapper's intentions. The lighting should match the time of day. There should not be many man-made lights along the coast, however, so consider that when choosing the time of day and ambient lighting conditions.
Docks and piers are a reoccurring motif along the coast, along with grounded and decaying vessels. As noted above, be sure to locate such props in a manner that indicates that their original positions at a higher sea level. High, rotting wooden piers are also good to create tension, as a player is likely to be afraid of falling off of it into the dangerous sands below. They also create "combat tunnels" where the player is restricted as to how much room they have, despite being in an otherwise open environment. Be sure to provide crates or other objects as cover during a firefight situation such as this.
Terrain should be entirely displacements, so be wise about visibility. The coast curves considerably, so naturally curved tunnels and caverns can be expected. Rocky coastlines are preferable as opposed to soft, sandy coasts, since the large boulders tend to obscure large stretches of coast which would increase the rendering load on a player's computer.
Remember, the water is infested with leeches. Falling in can be an instant kill, but along lower sandy coasts should only damage the player slightly. If the player is provided the buggy, a situation can be set up where the player must jump a section of water, adding to the excitement of a level.
The Combine is a relatively uncommon enemy here, as they have little use for most areas along the coast. Larger enemies such as striders would not fit in the atmosphere, however npc_antlionguards would be perfect. Allies can be included if the mapper wishes for larger battles. And, of course, there are the regular antlions, which should be set to spawn in the soft, loose sand. If the mapper wishes the player to control them, be sure to set the correct variable and provide the appropriate weapon_bugbait.
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