Travis,
I just put one in my bilge. I also have a 2005 nitro nx882 with the 150 merc. I suggest that you buy the thru the hull transduce with the water temp probe. With the transducer comes a little epoxy kit that you mix up. More than enough to do the job. I hooked everything up and just put the transducer in the bilge area. I took the boat boat out and hung the transducer over the side to get a depth reading. Then I put it in the bilge area with a large ziplock baggie over it so it wouldnt move. I kept moving it around and then ran at different speeds until I got good reading for speed and depth. When i got home, i removed the baggie, drew a little circel around the puck and then cleaned and dried the bilge. Follwed instructions from the epoxy kit and installed it. Having the same boat and hull as you, the spot I found was about 10 " forward of the plug and off center about 2". There is a stringer that runs down the middle of the bilge area, that is why it has to be off center a little. Works great at any speed. Probably lost a little sensitivity going thru the hull, but I have not noticed a problem spotting fish. I mounted the temp probe about 1/4" from the bottom of the hull right next to the jackplate mount. That works great also.