If the voltage goes to zero when connected to the starter, either the battery is bad or the connections are still dirty. Put the voltmeter on the battery post themselves and see what the voltage is after connecting the jumpers. If the battery post reads zero voltage, it is likely a bad battery or a massive short in the starter. That massive of a short in the starter would be unusual, but it is possible.
Does the starter spin by hand?
To be safe, I would jumper from the car battery, when the motor is off. If the motor cranks, get another boat battery.
Cleaned all connections again, tried marine battery, volts dropped to zero when connected to ground and starter post. Used car battery, sparks at starter post when touching but no spiny spin.... Guessing battery is bad as well as the starter needing some love or replacement. Pulled it off, we'll see what I find.
Thanks!
Lee