I just finished installing a Faria Air/Water temp + Depth gauge. All it required was adding air and water temp sensors, and I was able to use the existing depth transducer that was with the original depth gauge. So only drilling one hole through the transom was required.
The instructions are here:
http://faria-instruments.com/site_manuals/IS0197C_ecr6190.pdf
Original:
Upgrade:
Now, it depends on the year/style of boat you have, but you need to get access to behind the dash to take out the old gauge. For my 2007 H220 I had to take out the stock Clarion radio. There were six screws.
Then I used a 1/4" ratchet, size 5/16" to take out the 2 nuts behind the existing gauge.
The old depth tranducer was simply plug and play. For power, I had to splice the purple and black wires. Purple = +12V on with ignition, Black = ground. I also wired in the air temp sensor at this time. When you do this be sure to run all of the wires through the backing of the gauge before you solder them in, not just through the hole in the dash. (Yes, I'm speaking from experience.)
Now, I had to figure out a place to mount the water temp sensor. The instructions don't give much instructions other than, "Mount the water temperature sensor as low as possible on the transom."
I then ran the wire all the way back up to the front, along with the steering cable. I then zip tied the excess wires underneath the dash. A picture of my soldering ability.
Now the fun part was getting everything to go back into the dash, then getting the gauge backing back over the bolts and putting the 5/16" nuts on.... But it worked out fine.
Finished installation:
If you want any more pictures, I'd be happy to take them. I can also try to answer any questions if you have them.
Also, I have my old gauge for sale in the for sale forum if anyone is interested.
http://www.smwebhead.com/phpBB3/viewtopic.php?f=8&t=3752