A good project worth doing...I had one on my 29 Blackfin, and am installing one on my 282 Sailfish. Elevate the float switch so that it sets higher than your bilge pump float switch, but 3 or 4 inches. That way it won't sound off every time your bilge pump kicks in, but will sound off if the water level rises that 3" - 4". Make a horizontal platform for the sensor to mount on, and attach it, using good installation practices; e.g. resin-coated marine plywood or a piece of King Starboard (or go to the dept store and buy a small plastic-type cutting board. Use a piece of aluminum angle (Lowes) to attach the board to a stringer. Coated screw threads with 3M 4200 or 5200. Make sure your installation and wiring does not interfere with the regular bilge pump/float switch.
Grady's all have wiring chase-tubes that run forward from the rear bilge area, and can be accessed under the helm area. Use marine wire (tinned conductors), not something from Radio SHack or Auto Zone. Install your warning horn at the helm, and properly fuse it near the battery, and connect to battery + (not through the battery disconnect switch). This way the alarm will sound if the bilge is filling with water and the battery switches are off. That will give some audible warning to persons at the dock, when you are not around.
If you are unsure of how to do any of this, I would suggest you enlist the aid of someone who is familiar with this type of work.
I think Brian has installed one on his boat, and maybe can weigh in here with some advice.
