There are two tools I just can not get along without on the boat. The first of course is a screwdriver. A multi-bit style screwdriver with a complete set of straight, phillips, and the square drive bits so commonly used on boats. There is a full kit of tools aboard the Nordic Quest, usually everything you need, But it is the screwdriver that I grab first… You need a screwdriver to do just about anything.
The second critical tool is a DMM, or Digital Multi-Meter, commonly called simply a meter.
Checking for power, testing batteries, checking a cable for a break or short circuit, the DMM is critical. Properly used a DMM will give you a view into the electrical goings-on. Electrical circuits do not need to be as mysterious as black magic, they can be worked on. As you can not see electricity you need something to make it visible, to measure it, you need a DMM at a minimum. There are more advanced instruments, oscilloscopes, logic analyzers, etc., but the basic DMM can be found in any electronics lab and in every toolkit.
On the boat or on-the-job, I always have a DMM at-hand, I am an electrical engineer after all. I believe everyone should be as familiar with using a simple DMM as they are in using a screwdriver.
While a good Fluke or Agilent meter will set you back a few hundred dollars, a decent meter can be as little as forty or fifty dollars. There are cheap Chinese made meters for twenty or less, they usually work, but can be suspect with regards to safety on high voltage or high current circuits. Probably fine for working on the twelve volt circuits found on a boat. The meter in the toolkit on the Nordic Quest has helped solve the problem more than once.