Pretty sure you have a Faria depth gauge. Is the gauge dead with no read out and back lighting or just reading wacky?
http://www.faria-instruments.com/depthsounder.php
You can either call your dealer or Four Winns to get the Faria model. Go on website to get the connection diagram. It may be in your manual but I'm on road so can't check mine to see if it's in there.
Do you have voltmeter (and ideally a little DC power supply?).
Start at connection under the dash. Pull it off an reconnect to make sure you have solid contacts. If connection is good, it's either wiring, transducer, or guage.
You need to get the model number to find what voltage input the gauge is to see if you can get it to read over the range. You need DC power supply to do this. Without knowing voltage range not sure if a C or D cell batter will work.
To check the wiring between gauge and transducer you need to check contunuity. Short one end of wires together and measure resistance at other end. Should be near zero. If you get easy access to transducer you can get good look at it to see if wiring is bad, corroded, etc. Sometimes just cleaning contacts and connections gets it working again.
Hard to give you step by step without knowing how savvy you are with troubleshooting wiring. If you take pictures we can walk you through it. Not hard. Cheerio....