"Could" is used to describe something that is conditional: it may or may not happen/ed (just a guess or imagining what could happen in the future or what might have happened in the past). For example: "I could have bought a house, but I chose to buy a boat instead." In the end I chose the path of buying the boat, rather than a house. By using "could", it implies I was capable of buying either. "Able to" and "managed to" is used to describe your potential (in the past) or what you were able to achieve. Like the examples in your book: "The boat was able to the reach port."/ "The boat managed to reach the port." In the end, the boat made it to the port because it was capable of doing so. In colloquial speech, I personally use "could" in place of "able to" and "managed to" all the time("I couldn't do it."/ "I couldn't even look at it."), but it's not very academic or formal. So I guess don't use it for formal writing purposes. I hope this helps.

was able to ：used in past tense，things actually happened in the past could： used in subjunctive sentence，things did not actually happen or might happen in future