Boat radios and audio
This category holds boat head units and speakers, that is audio on board and not radio sets for traffic. On the speaker it is cutout diameter and mounting depth that decide, because the depth behind the panel is not something you enlarge. On the head unit the RCA output matters most, because that is the only way an amplifier joins later. And after that it holds that you compare RMS and not peak power, and that impedance caps how many speakers one channel takes.
Boat radios and audio
This category holds flush-mount head units and the speakers that go with them, that is audio and not radio sets for traffic. They differ from a car radio in having to take sun, salt and spray, and that shows in the ingress rating in the model name. The supply is twelve volts as in a car, only the fitting is different, because you drill into laminate and not into sheet metal. Amplifiers and wired remotes at the helm belong here too, while radio sets, chartplotters and sounders do not.
The first thing you measure is cutout diameter and mounting depth. Speakers are quoted by the diameter of the hole, usually around a hundred and sixty millimetres, and alongside it by the depth behind the panel. The hole can be opened up, the depth cannot, so it is the depth that decides whether the speaker fits. On the head unit it is the same, because besides the slot you need room behind it for the plugs and for the cable, which must not kink.
The second is sound sources and the RCA output. Bluetooth sorts out the phone, USB a stick, and the FM and AM tuner picks up whatever is broadcast from shore. What is decisive, though, is the RCA output, because that is the only way an amplifier and a second pair of speakers join later. Without it you are left with what the head unit manages on its own, and that is rarely much.
On power the first number you meet is RMS against peak power. The box carries peak in large figures, that is what the unit survives for a fraction of a second, whereas RMS is the continuous power and only that is worth comparing. It is sensible to keep the amplifier roughly between eighty and a hundred and twenty-five per cent of the speaker RMS. Below that mark the amplifier slips easily into clipping, and the speaker is no better for it.
The last is impedance and the number of speakers. Boat head units are almost always built for four ohms, and each channel counts on one speaker. Hang two in parallel on the same channel and the impedance drops by half, the unit runs warm and eventually shuts itself down.