First remove connectors from battery and thoroughly cleanbattery posts and connectors. Then with engine is running at 1000 rpm connect avoltmeter to the battery terminals. You should be getting between 13.8 and 14.6volts. If so, then you have a problem with the gauge, gauge wiring, or ECM.
Remove gauge and thoroughly clean connectors. While gauge isremoved, direct wire it to a power source (boat battery or separate battery)and compare its reading to your voltmeter. They should read the same +/- a halfvolt or so. If not, replace the gauge. If they do read the same re-connect thegauge and retest. If it is still reading too low, then you have a problem withthe wiring to the gauge, or the ECM (electronic control module.) The usualproblem is that the gauge ground wire gets corroded, so check that first. Ifstill not right, disconnect the signal (S) wire from the gauge, and with enginerunning at 1000 rpm, use your voltmeter to test the voltage between the signalwire and a known good ground terminal. (Note that the voltage coming out of theECM may be a bit lower than the voltage coming directly off the alternator tothe battery – I have no idea why that is, but on my 225 the alternator voltagereads 14.6 and the ECM voltage reads 13.8.) If you are still getting a lowreading, then you have a problem with the ECM or the wiring from the ECM. Atthis point, you will need to do some testing on your ECM, so hold that offuntil you determine that as the problem
If you are not getting 13.8 to 14.6 volts at the batterywhile engine is running, then remove connectors to the alternator, thoroughlyclean and reconnect and re-test. Not sure how your 130 alternator is set up,but if it has a separate ground wire, test the resistance between the batterynegative terminal and the alternator ground terminal, it should be very nearzero resistance. Test the resistance between the battery positive terminal andthe alternator positive terminal. It should also be very near zero resistance.
If you still have low output from the alternator, then you likelyneed to replace or rebuild it.