Port and Starboard
Every boat is different, but if inside, down in the basement, usually on each main bulkhead facing aft into the salon. Or, if it makes sense, on each side of the companionway facing forward. If in the cockpit, possibly on the verital surfaces (each side) between your butt and feet (behind your calf) or on the inside of the transom P&S. But, it all depends on what is on the other side, how deep the dead space is, how you can run wires, etc. Measure twice and cut once as they say. Speaker boxes (which help the sound) on brackets might be easier to install, but might also stick out too far. And they are nice if you don't feel comfortable cutting a big hole in a major bulkhead, etc. In the cockpit have it high enough so if you get pooped or your scuppers clog, you won't take on water through the speaker holes because they are too close to the cockpit floors. Lots of things to think about and that is a good thing to do before cutting away and then resenting it.