car radio for boat

[Seakindly]

"As a first step I would measure the draw. A dc clamp multimeter can measure this without even having to disconnect any wires"

Looks like a useful bit of kit. More useful than a standard MM on a boat?

 

[noelex]

. More useful than a standard MM on a boat?

Yes.
Most will also work as a standard multimeter. The clamp models are dearer however than a standard multimeter. If buying one check it can measure DC amps some of the cheap ones will only measure AC amps.
As an alternative you can get just the coil / clamp part as an adapter to plug into a standard multimeter.