In volume 3 of Shebly Foote's book on the American Civil war, he mentions that once out to sea, Winslow got a message from France about Semmes's intentions. Does that mean there were boats going back and forth from ship to shore before the battle? I know there were some Yachts near the battle so they could watch. Would they have been the source of communication?
I'm confused by the phrase "once out to sea," and I don't know what Semmes' intentions were, other than to fight, but I suspect Winslow received the message on shore, after taking a boat to shore from his ship the Kearsarge.
For context: Winslow and the Kearsarge had been pursuing Semmes and the Alabama for about 2 years. The Alabama arrived at Cherbourg on June 11th, intending to dock for resupply/repair, but the dry docks were full and he was denied. Kearsarge arrived at Cherbourg on the 14th without docking.
According to the Kearsarge's logbook, Winslow sent a boat ashore but the boat was not allowed to land; a second boat with a more senior officer was permitted to land (p.309). On the 15th, Winslow "went ashore... himself to find out how things stand" (p.310) and returned with news that Alabama was planning to fight. The author of the logbook reports that Winslow "had received an abstract of a letter sent by Capt Semmes to the Confederate Agent" stating he planned to fight ("if the Kearsarge would wait for him he would not detain her but a short time," as the logbook's author put it).
On the 19th, Kearsarge saw Alabama leaving port; both ships sailed out to international waters (to ensure the fight didn't happen in French waters, Alabama was escorted by the French ship Couronne) and battled for about an hour.
After Alabama was sunk, Semmes escaped aboard an English yacht, the Deerhound, which had putatively sailed out to watch the battle but turned out to be an accomplice of Semmes that spirited away Semmes and 40 of his officers and crew, preventing Winslow from taking them prisoner.
I don't know what was used in this particular case, but I can tell you what was available at that time. Besides a courier sent by boat.
a) on small distance: flag communication (a person with two flags in his hands, see Flag semaphore. Used on short distances.
b) flags raised on a mast, Naval flag signaling. Used on intermediate distances. Frequently used in a battle, or in convoy to transmit commands. Used on intermediate distances.
c) Communication with special lights: a searchlight specially constructed so that you can dim it or open quickly with a kind of the blind in front of it. Morse code or some other similar code can be used. Advantages: it is more secure, because you direct your searchlight narrow beam at the recipient. Works especially well when it is dark. (I don't know English name of this system, in Russian it is called "ratier", probably after the name of the inventor.) The range is limited only by the horizon, but you can place the searchlight high, for example on a top, to achieve quite a long distance. This system has the longest range of all optical systems.
To increase the range, sometimes a signal was transmitted through a chain of ships, two nearby ships or boats within the range of any of the previously described system.
Remark. A searchlight does not necessarily use electricity. Lighthouses exist since very ancient times, but I don't know whether non-electric "ratiers" were ever used.