I would like to assosciate myself with OMFG's comments. These topics run right off the rails when someone speculates on some minute point and then that becomes the topic; we construct a resume of the person with no available facts.
The average boater probably has little or no knowledge of Securite, Pan, or Mayday broadcasts. About all that can be hoped for is that they'd take a USCG-Aux course. Most will not spend the staggering amount of time necessary to educate themselves, and will prefer to learn as they go. Them's the facts.
If you have any hopes of re-using any of the electrics, you'll use a CO2 extinguisher on the fire. Dry chemical destroys the wiring. Foam extinguishers, which only use CO2 to produce the foam, not as an extinguishing agent per se, are only suitable for oil fires. The foam floats on the oil or water and smothers the fire. They are ineffective on electrical fires and unsafe for that use. Dry chemical should be used with caution on a boat. The dry chemical may make the boat uninhabitable post fire and on staying on deck may not be an option. CO2 works quite well for most fires encountered on board. Given the nature and size of boats, if a fire get's out of hand, with limited fire-fighting capabilities, it makes perfect sense to button up the cabin and use smothering or oxygen deprivation to extinguish the fire. Generally on board ship, the opposite of shore-side practise is employed. Shore-side fire fighters tend to want to ventilate the fire so that manpower can safely get to the source of the flames and so as to reduce the obscuring smoke. On board ship, ventilation is immediately shut down to prevent the travel of fire and to begin to deny it oxygen. When a fire get's out of control on a boat, it might be wise to button her up and discharge the as much CO2 into the sealed compartment as possible. While this technique may not put out your fire as quickly and effectively as fighting it directly with an unlimited supply of extinguishers, it may well prevent the fire from burning the boat to the waterline and prevent the contemplation of a very, very long swim.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.