lots of GHits

WEST INDIES

The West Indies is a large group of islands that separate the Caribbean Sea from the Atlantic Ocean. They comprise three main island groups:

Bahamas (north)
Greater Antilles (central)
Lesser Antilles (southeast)

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Note added at 2 hrs (2008-11-17 19:00:24 GMT)
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Ask a good British schoolchild where Jamaica is and the answer will be in the West Indies - not in the Caribbean Chain or Arc.

The specific translation "Antilles" refers to the arc of Greater Antilles to Lesser Antilles islands beginning with Cuba and ending with Trinidad. The more general translation "Caribbean Basin" refers, on the one hand, to the geographic area of the continental land mass from, on the northern side (the U.S.), the western side (Central America) and the southern side (Colombia and Venezuela), and on the other, the abovementioned arc of Greater and Lesser Antilles islands.

sources:

Antilles - Wikipedia, the free encyclopedia - The Antilles (the same in French; Antillas in Spanish; Antillen in Dutch) refers to the islands forming the greater part of the West Indies in the Caribbean ...
en.wikipedia.org/wiki/Antilles - 39k -

Caribbean Basin - Wikipedia, the free encyclopedia - The Caribbean Basin is generally defined as the area running from Florida westward along the Gulf coast, then south along the Mexican coast through Central ...
en.wikipedia.org/wiki/Caribbean_Basin - 17k -

This is obviously not a direct translation of "Arc des Caraibes" but I agree with kashew that "Caribbean arc" is a bit too scientific a register for an insurance policy (to the point where I don't think the meaning is clear - though not sure if meaning is clearer in French).

Hattie's clarification makes it clear that this is not quite just another way of saying "The Caribbean", because it excludes the mainland countries of the Caribbean region/West Indies (the Guyanas and Suriname): "Arc" has been used to pick out the islands rather than the mainland (probably different insurance risks apply) and so I'd translate it as the Caribbean archipelago (ie island group) or islands.

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Note added at 8 hrs (2008-11-18 00:46:24 GMT)
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Actually, since you say the Guyanas etc. come under the same coverage territory, it sounds like they're just enumerating the parts of the West Indies - the islands on the one hand and coastal territories on the other - to distinguish from, eg. Central America, which might fall under a different heading.