Learn French: Le garde-côte doit veiller toute la nuit. - The coastguard must keep watch all night.

While regarder means "to watch" in the sense of looking at something (like a TV or a bird), veiller specifically means "to stay awake," "to be vigilant," or "to keep watch." It is used when someone is staying up to monitor or protect something.

How do you make a compound noun like garde-côte plural?

It becomes les garde-côtes. When a compound noun is formed by a verb (garde, from garder) and a noun (côte), the verb part never takes an -s. The noun part usually takes an -s because the guards are watching over multiple coasts.

What is the difference between toute la nuit and toutes les nuits?

Toute la nuit means "all night long," focusing on the continuous duration of one single night. If you wrote toutes les nuits, it would mean "every night," focusing on a repeated habit.

Is garde-côte always masculine?

As a general profession, it is masculine (le garde-côte). If you were referring specifically to a female coastguard, you would use la garde-côte. Note that when garde refers to the action of guarding rather than a person, it is always feminine (like la garde d'enfants for childcare).