I have a 3.0 Mercruiser. None of my gauges are working. 4 of them, fuel, temp, oil, and battery peg out when the ignition is on and all of the others do not move at all. Any ideas???

check the grounds and make sure they are all good.

can you put that into "city boy" terms for me?

start at the battery and clean and redo the negative cable.
did you do anything recently to the batterys, wiring?

sounds like you blew a Fuse somewhere

I checked all of the fuses under the dash and they are all fine. I took the wires off the battery terminals and cleaned them and put everything back on. I would think that the battery is fine because the boat has no problem starting.

With the key in the on position the temp, trim, oil, and fuel gauges all peg out. The other 3 gauges dont even move. When the key is off and I activate one of the auxiliary switches or press the horn button the "buzzer" goes off and those same 4 gauges peg out. The other auxiliary switch, the blower, the bilge, the depth finder, and the radio all work fine.

The only thing that I have done to the boat lately is I changed the drive unit oil yesterday.

Sounds like you have the battery cables on backwards. Like the Red cable is on Negitive of the battery and the Black cable is on Positive of the battery.

Red is on positive. The battery has never been touched at all (other than me cleaning the connections this morning).

Somewhere in the system you are getting a back feed.

yep got a positive feed to the grounded side, just needs to be a little to cause a problem. start with the guage wires under the console, since some of them move and others don't check all the wiring and make sure that none of the pos-neg are touching/bare.