If the gauge is pegged as soon as you turn on the key switch, then this is likely a short to ground someplace in the sense wire or the sensor is defective.  Failing this, while it's possible to measure either voltage or resistance at the sensor when then engine is hot, I don't have a chart that shows what these readings should be and one of the problems with this technique is determining what a hot engine is when you suspect your gauge to begin with.

So, one sure way to determine if the temperature measuring system is working properly is to remove the sending unit, heat it to a known value, and then read the panel gauge.  Here's how:   
https://www.catalina36.org/forum/technical-discussion/problem-w-temp-gaugesensor

If there's a significant error, then you have to determine which component is in error.  It's a shotgun approach at this point, with the sensor being the #1 suspect.

When you reinstall your sensor, please make sure you "burp" your cooling system when you top it off with coolant.  This procedure is covered in the forum.

Nick,
Re: "When you reinstall your sensor, please make sure you "burp" your cooling system when you top it off with coolant.  This procedure is covered in the forum."
Where do I find this burp procedure in the forum?
Thanks
Steven Jones
Past Mainsheet Editor C36IA
C36 #2164  2003
 

The level of burping depends on how much coolant was removed:

All coolant was removed, including water heater lines:
Engine cool and not running.  All coolant petcocks closed and engine coolant cap removed.  You'll need a funnel that will fit into the end of 3/8" hose.  Locate the 3/8" hose going to the bottom of the thermostat housing, loosen the hose clamp, and pull the hose off the nipple.  Hold the removed hose above the thermostat housing and using the funnel slowly add coolant into the hose until coolant comes out of the nipple that you just removed the hose from.  Once that happens, just put the hose back on the nipple and tighten up the hose clamp.  Go on to next procedure.

Partial coolant removed (engine block, heat exchanger, but not water heater):
Thermostat petcock open, all other petcocks closed, engine coolant cap removed.  Slowly add coolant through the engine cap until about 1" below the top of the filler neck or until coolant comes out of thermostat petcock.  Leave the filler cap off, close the petcock, and start the engine.  Let the engine run for a few minutes while monitoring the coolant level, keeping it at about 1" below the filler neck.  Monitor the temp gauge and the temp of the coolant with your finger.  Once the coolant starts to feel warm, quickly rev the engine a few times to help dispel air.  Once the coolant gets hot or the coolant level starts to visibly rise (10 to 15 minutes) again quickly rev the engine a few times and then reinstall the filler cap.  Add coolant to the overflow reservoir to the full line, allow the engine to reach normal operating temp, and then shutdown. Abnormal indications would be the temp gauge reading too high or too low for a given run time.  After cooling, it would be a normal and good indication to see the overflow reservoir level drop just a bit, stabilizing after a few engine runs.

I couldn't agree with Nick more. Don't assume it's a gage issue until proven otherwise. I have used in the past a Infrared Thermometer. Just point it at the sending unit or thermostat housing. Quick and easy. I've never used it on my catalina, but I have on others. It will close, within 5 degrees or so. They are $15 to $100. Fun tool, I use it for all kinds of things.
http://www.amazon.com/Etekcity-Lasergrip-Temperature-Non-contact-Thermometer/dp/B00K5QVBCU/ref=sr_1_9?ie=UTF8&qid=1436827727&sr=8-9&keywords=infrared+thermometer(link is external)