tachometer died on 93 ski nautique

Are you sure the gauge has gone south? Not much in a tach to go bad. Check for loose connections or a bad wire. Is there continuity between the distributor and the tach? Tachs do not have a sender per se, they pick up the firing pulse from the positive lead off the distributor, usually a gray wire.

Seriously doubt if VDO still markets that tach. Most of the gauge outfits change their lines every few years because the boat manufacturers need variety in the lines to attract new customers. Contact them directly through their website.

As for replacing a tach, hardest part is crawling under the dash to get to the back of the unit.

93 nautiques are notorious for bad fuel gauges and spiking temp gauges. both of mine are bad. so it could be the tack but check for wiring. check the whole series as the gauges are intertwined with each other on the back. might be a good time to justify perfect pass as you get a digital tack read out that is much more accurate than the vdo. check with ski dealers , they take a bunch of perfect pass out of boats traded in and sell seperate for maybe 500.00 $