See the blue plug, the temperature sending unit is normally screwed in that hole, with a tan wire attached to it.

Like mentioned, shoot that area with an IR thermometer

Sounds like it might be a springtime thing since you're winterized 

The picture will make more sense if you rotate it 90 degrees clockwise  

+1 on what Tim says.  The I.R. temp gun is a great tool.

You also may have a temp sending unit problem.  In my 1994, I had a couple of water temp sending units go bad.  At first, I didn't use the I.R. temp gun as I should have done, but instead, I started throwing parts at the engine (thermostat and impeller).  No joy, the temp gauge still showed 190-200 degrees.  I changed the temp sending unit and the gauge showed a rock-solid 160...for about 1 season.  Then, at the beginning of the following year, the temp gauge again was showing 190-200 degrees again.  This time, I had the I.R. temp gun in the boat.  I ran a series of temp probes with the I.R. gun and it showed that the intake manifold area near the temp sending unit was running between 155 and 165 degrees.  So, I put in another new sending unit and the temp gauge stayed at 160 degrees until I sold the boat.

JQ

As coached, use the IR Gun, one of the best tools you will add to your garage.  You will find you use it for many things once you own one.   Checking brakes, Exhaust manifolds, Home AC performance and your engine cooling system.  A great tool to have.

The sending unit feeds both the gauge and the alarm box under the dash.

The resistance range of the sender needs to be matched to the gauge and alarm box, otherwise your readings won't be accurate.

It should work right.

Right now the gauge and the alarm box are both getting the same high temperature signal from the sender

One thing that could be checked is the 8 plug connector at the back of the engine to make sure all the plugs are clean. 

The tan wire is the one that feeds the alarm box and the gauge.

One thing to watch out for is that if that 8 plug connector is all corroded and ugly looking, you might cause more problems than you have now if wires and connectors start falling apart. as you wiggle or pull it apart.

If that's the case, just consider yourself lucky that you found the corrosion or wire issues before they stranded you somewhere and make some repairs to ensure good connections. 

Mark

Here's a picture or 2 and some discussion of the connector.

Like Uncle Buck's boat, you may have only 7 wires on the boat side since you have a voltmeter on the dash.

A broken wire will make it peg low, not your case either.

But corroded conditions or a couple of wires making contact that shouldn't be can cause just plain wierd issues.

You could check the power, ground and the sending unit connections at the gauge too, to make sure they're tight and clean, but your problem seems to be sender or wiring running up to the gauge/alarm box since the gauge reads 200 ish and that about where the alarm goes off. (about 205 degrees) 

And 2 senders reading about the same makes me think wiring/wiring connections are causing the high reading.

This is like a good murder mystery, lots of twists and turns and new info 

Assuming your 89 has VDO gauges, your temp gauge probably has a range of 100 to 220.

Like you said it should go downscale when the boat is off, not upscale.

If your gauge is pegged high before you start the engine, what happens to it as soon as you start the engine, does it drop down to the low end and slowly come up as the engine warms up? Or work it's way down to 200 ish from the high peg at 220

A couple of things you could do are

1) using a wire with a couple of alligator clips, hook one end to your temperature sensor where the tan wire attaches, (no need to remove the tan wire) hook the other end to a good clean ground on the engine.   Now turn the key to run and see what the gauge does.

Normally with the gauge at the bottom peg, when you have the sender grounded with the test wire, it will peg high as soon as the key is in Run.

From the sound of things, yours might go from upscale to downscale, what yours does, would be good info to know.

2) after that, remove the test wire and unhook the tan wire , turn the key to Run and describe what your gauge does. Normally it would be pegged low and it would stay there

Those things will tell you if your gauge is maybe broken or wired wrong

And like a good Columbo mystery,,,,,,,,,,,,,just one more question , have you checked the connections under the dash that you cut and crimped a few years ago?

And maybe one more, what does your voltmeter on the dash read when the engine is running?