Think of the line where the land stops and the sea begins, and of people whose whole job is to watch that line. Looking out, they see a boat that is drifting or a who has gone too far, and they go and bring them back. Looking in, they see what is arriving on the shore, and they check it. One word covers both the body of people and any one of them.
The source gives it as coast + guard. The two parts already name the job: someone guarding the coast. That is why the word works equally well for the service and for a single member of it.
來源說這個字就是 coast（海岸）加 guard（守衛）。兩個部分合起來已經把工作寫在名字裡：守著海岸的。也因為字面講的是「守海岸」這件事，它既可以指整個組織，也可以指做這件事的其中一個人。
Wiktionary, "coastguard"
One branch
守海岸的隊伍與隊員
登入就可以看這一支怎麼連回核心畫面。
coastguard service · noun
The Coastguard is the organization that watches the sea along a country's shore. It helps swimmers and ships that are in danger, and it also works to stop illegal activity around the coast.
海岸巡防隊