B, b, b, but, Q..... How can an expanse of water with many scattered islands (as per your N.A. Dictionary) be covered with little green islands? The little green islands are already a part of the archipelago by definition, eh? Even the OED says an archipelago is water with islands, not just water which little green islands can cover. Therein lies the rub.
On the other hand, although I agree with Merriam Webster's Collegiate, I never said an archipelago was merely a group of islands. The archipelago where I live is many islands as well as many bays, inlets, harbors, coves, etc., near the mouth of a geographically large bay, opening to an ocean. As such, I defend my statement that an archipelago cannot be covered with little green islands. :-)
Semantics.... gotta love 'em, eh?