First remove the tach and clean the contacts and the ring terminals.
There is a dial on the back of the tach and it should be pointing to the "6P" position, move the dial around to clear the static and put it back on 6P, then reinstall the tach.
Second: Make sure battery connections are clean and tight, and inspect the gray,red, and yellow wire bullet connectors at the voltage regulator. Make sure they are clean, show no signs of excess heat, and are snapped together tightly.
Third: Take it out for a few short runs, and if the tach starts jumping around or not registering then take notice of what the voltmeter is doing when the tach is acting up ? If it is spiking above 14.4v or staying down around 12v then the outboard needs charging system troubleshooting.
If voltage is steady in the 13.5 range then replace the tach.