Electrical question boat has brand new deep cell battery the guage is reading at the most on the water 12.5 volts I have checked all the connections and they are good. I starting noticing it when my radio started cutting out when the boat was on or when it was off just a stock system by the way . Any suggestions would be great

What is a "deep cell" battery??? Deep cycle???

Check the battery voltage with a real volt meter and also check the alternator's voltage output.