Low Voltage

[DBPEteacher]

New member here.  I just bought a '95 malibu sunsetter with a new engine, battery and tune up from a guy that worked at mastercraft and he said they checked everything out before selling it to me.  I took it out one time and it died the first time out.  It started back up about 10 minutes later.  A few days later I go to take the boat out and I turn the blower on and I don't hear it (works, but doesn't seem to have the power as I remember)  I try starting the boat and it doesn't even crank.  There is no kill switch, throttle was in neutral, and it shows only getting ~8 volts on the dash, while the battery reads fully charged.  I tried the stereo and it turned on for a few seconds and then cut in and out.  the horn didn't work, nav lights didn't work.  I am guessing it is some type of wiring issue but I am not exactly sure what it could be.  I don't see any disconnected wires and the wires to the battery terminal are clean.  Any suggestions would be greatly appreciated.