"If the drop is measured using
"If the drop is measured using the battery positive as the reference point and you're measuring to various other points at the dash, you'll usually have some resistance in the main harness. You don't want a lot (less than an ohm) but it takes two to tango so you'll need to reference to the battery negative terminal and measure all of your grounds, too.
All power measurements refer to the battery posts, not the cable clamps. If you have resistance, clean the battery terminals, check the plug to the boat/motor harness and make sure everything is clean and tight at the ignition switch, power distribution and ground buss.
The buzzer will make no difference- it's a piezo-electric element and doesn't act like a dynamic driver, which is what a regular speaker is. The load is generally stable across the frequency range. That's not causing the drop. It's only in-circuit when the button is pressed."