When your boat is running you should be closer to 14.4, but just 12 with the motor off. the pre-out voltage will not hurt the amp. The 4 volt will be just fine. If you have RCA pre-outs for the front vs the rear speakers, then you should get a 4 channel amp that can do 50 to 75W RMS output at 4Ohms per channel. If you have just 1 RCA pre-out, then you can't fade anyway, (you will have just left and right control) so get just a two channel amp with 75w RMS per channel at 2Ohms per channel.
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