In the lowest part of your boat, most likely the stern on a planning hull, you'll find it. The high water float switch will probably be raised about 4".
I’ll try to get pics today, but mine is mounted on a bracket about 4” up like NYCAP mentioned above my mid bilge pump. Just aft of the small door going into engine room from inside of boat.
This is where I figured it would be but don't see one there. I had a leak on my Raw Water Cooling pump. This boat runs bow high so I had water collect at the gas tank bulkhead. This caused the boat to ride lower in the water and drink more fuel. Collecting more water. No alarm. The surveyed never tested the alarm and I did not think of it either. I am now thinking of adding a bilge pump and the alarm float just aft of the batteries. Not sure how much water we had in the engine compartment but I know it was a bunch. We were running at night so I could not tell how low we were running. It was a good night, we made it back to port.
From the sounds of what you say adding a bilge pump with a high float switch connected to the alarm sounds like a good idea. Also check your limber holes in that bulkhead. I suspect they're blocked and the water should have gone aft to where you'll find your high water float switch.
I could not find any Limber Holes in the aft bulkhead. This is the bulkhead that separates my engine room and my fuel tank. Was not sure if this is a by design or a mistake. As Captain Chase said, my fore pump is just at the back of my freshwater tank and the bulkhead separating the cabin and engine room. The Mid pump is just on the other side of that bulkhead. The aft is all the way back. I have a Limber Hole between the cabin and the engine room but nothing between the engine room and the aft of the boat. Maybe it's there and I can not see it. I did try to shoot water and got nothing through. I will soon be an expert on this boat. LOL. Heading back to the boat tomorrow. The saltwater wiped out my starter solenoid. Need to get that repaired to see if any damage was done to the starter. Then maybe I can find the water leak. Oh, the joys of boat ownership!
You should have sealed bays under your motors to catch oil then on the center like a limber hole to allow water to pass to your main pump, but if you don't have bays you wouldn't have a limber hole as oil has to be kept from reaching your bilge pumps. If you do have bays look at the bottom of the bulkhead on the centerline. I never like the feeling of stepping into an engine room and finding deep water. I once had a shaft log leak and it's amazing how much water that lets in.
As my boat is new to me, did some searching...I have the 40 open... mine is at the aft end, above the rear bilge....
Found no float on mine. Even if I had one, horn was bad. Going with a 4 channel now that I have to add something. Also adding bilge pump run indicators since I’m running wire.