Koine Greek (from the Greek word for "common") was a variety of Greek spoken throughout the Greek world, including at the least the Greek Islands, Asia Minor, Egypt, and the Levant. The term "koine language" today refers to a lingua Franca used amongst speakers of related languages.

Koine Greek was based mostly on the Attic dialect, from the area surrounding Athens, but also included features from other dialects. I've read two different stories for its creation: it emerged in the seaport of Athens, or it emerged in Alexander's army. Either way it emerged as a way for speakers of different dialects to communicate with each other.

Setting aside the question of how different the dialects were, in 223BC a person from one part of the Greek world would've been able to communicate with people from other parts of the Greek world.

By definition, a group of dialects that undergo koineisation are different enough to encourage the creation of a koine, but have enough mutual intelligibility that the creation of a creole (which, to simplify, is a stripped-down hybrid of two languages) is not necessary.

Another question: I read in a historic novel set during the punic wars that "greek was the language of the sea", widely spoken by sailors all across the Mediterranean.

Is that true?