Engine noise in tower speakers

I finally got around to putting in my amp, sub, and tower speakers and I'm pleased with the sound except for the engine noise that comes through the tower speakers.

The amp is in the under-dash storage compartment of my malibu v-drive and I've run the power and ground cables straight back to the battery terminals. Yesterday some clown at Tweeter (electronics store) told me that the problem was that I was running the ground to the battery and the cable was too long, causing the excess noise -- he claimed to have done installs in "malibu boats" all the time and said he always grounds the amp to the "steel beams" in the boat -- well, we all know that is b.s. -- anyone got any real advice on what to do?