2003 SE Nautique 216 Electrical Issue

My brother recently helped me clean up the stereo wiring on my nautique 216 which was completed by the previous owner and it appeared to be a just get it done job. Once he was able to get it cleaned up (soldered all connections, new power and ground wire, etc.) it seemed to take care of a lot of electrical gremlins I was experiencing last year, however, I still have a few issues and I am wondering if I were to replace the power and ground wires from the battery to the dash if this would solve them. I just did replace my negative battery clamp coming from the engine with a clamp style because I noticed that bolt style that was being used was fairly corroded/greenish. I am planning on replacing the positive coming from the engine compartment with a clamp style as well just didn't have one with me over the weekend.    

Current Issues. - The boat appears to charge around 13.5 amps while under way. Sometimes I can turn on blower and nav lights and the volt gauge barely moves while other times I turn it on and it drops below 12V on gauge but I don't hear any warning sounds or see any lights. When it drops there are sometimes some small fluctuations in the temperature gauge as well. The more concerning issue is what I experienced yesterday while driving around I was cruising (less than 10 mph) and when I turned on the blower it started to killed the power to the boat, I quickly turned it off and was able to keep the boat running. After parking at a dock for a few minutes I started the boat back up with blower running and had not issues. I had a similar event happen last year while running the ballast pumps where the boat actually died when I turned them on.

Any suggestions? Do you think changing upsizing the ground and power wires from the battery to the dash will solve the issues? Thanks.