I used this to replace the factory horn on my Starcraft Islander 221B, which was an Aqua Signal 84400. Got tired of the "sqwaaaaak" sound that you couldn't hardly hear over the motor anyway. A horn is a safety item and equipping a new boat with such a weak horn is practically criminal. I'm amazed that Coast Guard standards even allow such a wimpy horn, and I worried I would fail one of the random safety inspections we're sometimes treated to at the boat launches around here. So I decided to upgrade, and my research led me to the Marco HR1.The Marco is physically quite a bit larger than the Aqua Signal. Fortunately, I had enough room in roughly the same location. I enlarged the existing small Aqua Signal hole to the bigger square shape required by the Marco using a sawzall, and in the process I relocated it in the direction where there was more room behind the wall. Just barely was able to make it fit. The Marco's grill is quite a bit larger than the Aqua Signal's grill so there was no problem covering up the old screw holes. Even managed to cover some scratches in the paint.For wiring, I used the supplied relay and triggered it by the existing horn wire. 10 gauge wires to feed the relay were run from the battery at the back of the boat up to the relay, with a 30A fuse near the battery.The result? The Marco is tremendously louder than the Aqua Signal. The tone is deeper than a typical car horn and is smoother than the old Aqua Signal too. The sqwaaaak sound it gone. It sounds like a horn, not some kind of dying waterfowl.Is it super loud? No, but it's loud enough. My previous boat had a Wolo and I think it was louder than this Marco. But it was mounted entirely external to the hull, and that makes a big difference. As far as hidden horns go, the Marco HR1 is probably the best you're going to do. I may supplement it in the future with something mounted externally, possibly a trumpet style, but the Aqua Signal to Marco upgrade was definitely worthwhile.