Coastwatchers played a significant part in the Battle of Guadalcanal. After the naval battles the Japanese tried to bomb the Americans into submission. The defending Hellcats, were no match for the escort Zeros unless they had a height advantage. The coastwatchers, predominately Kiwis, used to observe from the western Solomon Islands and pass on the information about the attacking aircraft. This gave the Hellcats time to take off and climb so that could dive and build up inertia to get through the Zero escort.
They used to be there on their own, totally reliant on the locals for food and also trust that they wouldn't let the Japs know where they were.