While cruising inland or near shore, a defective or failing boat/motor part isn't really a big deal...call for help, get a tow and order the part(s) you need for the fix or have it installed professionally. Offshore is a different matter and you'll want to carry spares to keep you out of trouble or perhaps spares that ensure your cruise will not end up being life threatening.
We were very thorough in stocking spare parts for an extended offshore voyage. After carefully stowing most of the spare parts, my gaze fixed on the spare starter motor for the sailboat's auxiliary engine. It didn't look right.
I decided I'd pull the working starter motor off the engine and see if I could fit the spare. I learned two things:
1) I didn't have the proper tools to get the running starter off
2) the spare didn't fit
At that point it dawned on me that a spare is no good unless it's proven. I then installed all the major spare parts and kept the working take offs as spares. In the process I learned that I really didn't know the ins and outs of an injector change and had to consult a manual which I didn't have on hand.
When it was all over, I was confident that the spares were all correct and functional because I had removed them from a properly functioning system. I also knew I had the tools and know how to install them.