Re: What spares?
double redundancy.
Look at things on the boat that if they fail (break, fall overboard, etc.), your boat fails, such as a fuel water seperator, a fuse or a key. Caryy a spare.
Look at things that can be bypassed if they fail and have materials for bypass: carrying hoses and clamps are a good example, also electric and bailing wire.
Think of things that can repair many problems--including tools (hammer, needlenose pliers) and materials (duck tape and cable ties)
Weigh the likelihood of failure, the consequence, and the practicality of carrying the replacement. For example: I don't carry a spare stern light becuase it is not likely to fail, it's not a big deal if it does where I boat, I can use a flashlight if I had to, and the spare would get in the way. I do, however, carry a battery powered bow light because the considerations I just listed come out the opposite. I also carry a spare bulb for the sternlight because the bulb is likely to fail, is easy to carry.
Assess your circumstances: I am not likely to lose an anchor and I can just go home if I do. A person on a long voyage would assess this issue differently. I don't carry a spare bilge pump on the small boat because even though I am many miles from other people I am never more than 500 yards from shore. On my large boat, I may be miles from shore, so it needs the extra pump. (Both have hand pumps and bailers).
I carry extras of lots of small stuff b/c I can: wire connectors, plugs, clamps, etc.
Most important: the stuff is kept in one or two watertight boxes, not scattered in compartments, pockets, etc. Those areas are for the spares of the spares.
