The tech who contacted you wants you to check the voltage that powers the radio with a DMM (Digital Multi-Meter). Its a good first step for troubleshooting anything electrical, but probably not what is wrong. Your charging system (if working properly) should be putting out about 13.2 volts which is nicely above the minimum of 11 volts he said that the radio requires but not enough to over power it. He also wants to know if you installed any other external amplifiers when you installed the radio.
It would help if I knew how old the engine is and whether it is an inboard or outboard engine. If it is an older boat it may just be faulty spark plug wires (or the old style that did not have radio interference suppression). There are also resistor spark plugs available that are RFI resistant.
Timbo correctly mentioned that it may be an RFI shielding problem, but usually the metal case on the radio shields the internal components. I am guessing that the antenna is picking up interference from the engine. It could also pick up RFI through the power wires to the radio. In this event, there are devices (called chokes)that can be installed on the power wire that will suppress the RFI. Another thing you can do if you suspect that the interference is through the power wires is to use "twisted pair" wire to power and ground the radio. Some guys will also run another separate ground wire directly from the engine block or negative battery terminal to the metal radio case. You can't ever have too good of a ground.
One thing that you can easily check is to rev up the engine with the radio on and listen to see if the static increases or decreases with engine speed. This is a good indicator that it is ignition interference.
Another source of RFI on an engine is the alternator but most modern alternators have suppression devices on them to avoid this.
I hope this helps. Please post again to let me know what you find out.