Shorter battery life and shorter starter life

I runn what are basically battery cables to my amp (but with finer wire strands)

Going to store to get new batteries, again.

Your Audio guy should have already known this stuff...Just Sayin

What is the reasoning for this? What ever current goes out the hot lead has to come back the neg lead, not more.
This is assuming you only have one +12v feed line.

Its to make sure no transient voltage can return on the ground side, Ive installed many high end switch gear over the years AC & DC and most of the manufacturer's recommend a 200% neutral or logic return ground, in the Telecom world, its just the way I adapt that line of thinking to other things

just fyi, you can find a welding supply and get grounding cable. Better quality and cheaper than audio stuff. Also the cable runs in boats are usually much longer than in cars. The cable needs to be much larger to deal with the distance, if not you create heat and large resistances.
Just as stated before. Your audio guy should know about all these problems. They are not unique to the boating world. I think it's time you find someone else, your guy doesn't sound knowledgeable.

Yeah your problem is likely in the cabling. If you have a big enough wire you wouldn't need a capacitor. There are plenty of us here with big systems that don't make lights fade when the bass hits and don't have capacitors.

Thanks guys, my audio guy deals in cars mostly but should be no different in a boat.

After taking in all the great information and going back to interstate batteries I have changed the deep cycles for 1000 cca starting batteries. The difference is amazing, the engines now turn over much quicker and the lights still dance to the music but it's 95% better than before. Again thanks to everybody for your input and advise, without the help from everyone here I would still be walking around in a fog.