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There are basically three grades of gauges I've found on boats. Well, four if you put the integrated system such as the VDO cluster or those clusters one finds on 80's era Bayliners...
-Low-end cheap plastic ones. These are typically not very water/moisture proof, and look as cheap as they really are. I had these on my '87 Galaxy 2100 and my first boat; a 1978 Cobia (complete with it's OMC 800 stringer and 350). They aren't very accurate (especially the tach), but I believe the reason the tach isn't so accurate is due to moisture getting into the electronics. Since there is a switch on the back that sets the # of cylinders for the engine, the corrosion causes issues with the switch. Some of these tachs also have a potentiometer to calibrate the tach. I've never seen one that was spot-on, but I've been able to easily calibrate it to make it spot-on (at least at one point in the range, which I usually set at 2800 RPM).
-Mid-grade plastic/glass/metal ones. My Vista came with these. My Signature 24 also had some of these made by VDO, but Chaparral decided to make that panel a little less-standard in that the gauges didn't have bezels; they were a nice attractive flush fit with a little bit of convex shape to cut glare, but were plastic and crazed a bit with age and UV exposure. The ones in the Vista had actual glass (it looked like it anyway) crystals, metal surrounds and decent looking faces. The electronics, however seem to be the same as what's in the cheap ones. I replaced the gauges on the Vista with a new set of these that don't have gold trim around them due to issues with a couple of the gauges not working. Had to calibrate the tach right out of the box; it was ~10% off from reality. This leads me to believe that most manufacturers simply slap the tach in the panel and hook it up without verifying calibration. Again, being not totally sealed, the tach is typically the one to act up first (there's more electronics in the tach than any other gauge).
-High-end gauges that easily cost the same amount for one gauge as a person can pay for a whole set of the mid-grade gauges. They are typically oil or glycerin filled, large, and attractive. Since they are filled, they are very well sealed against moisture. They typically have anodized trim available in every gaudy color of the rainbow. We all know that the more wild colors one can put on a boat, the faster it goes, so that seems to be why these gauges are most popular with the GFBL boat crowd. I swear they even consider the gauges' appearance in boat show contests. As for the accuracy, I imagine they're a bit better than the other styles, but I don't know for sure, nor do I ever plan to find out. They also tend to leak the contents of the gauge all over the place after a little age gets on them, garnering tons of anguish from their owners that a drop of oil has suddenly appeared on their holographic insert dash panel.
Ultimately, those gauges all use the same sensors to drive them, and looking at the longevity issues with leaky oil-filled ones, I can't see where one style lasts much longer than another.
I guess what I'm saying is either just tap the gauge from time to time, or replace it with whatever your wallet decides is best.
That being said, I am far from an expert on the subject; these are simply my observations.
If it were me, I'd just tap the thing from time to time, as I tend to use my ears as a tachometer more than I use the tachometer as a tachometer.
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