I have a spares/gear "log" in the form of blueprint-style plans of the boats from above, sequentially numbered. Accompanying this is a list of stowed spares/gear on the back, like this
(7) Port pilothouse underdeck locker: Engine spares/Lube oils/Rigging spares.
7a: Loose: Seven 4 L 15 W 30 oil jugs. 7b1: Boxed: Spare fuel filters, oil filters, secondary filter elements, engine belts. 7b2: Boxed: Lube grease, acetone, lamp oil, 3 in 1, naval jelly, Liquid Wrench, WD-40.
(8): Port Pilothouse tool locker under settee: (See exploded diagram)
(9): Port aft bulkhead pilothouse 10 lb. fire extinguisher
(10): Helm (see exploded diagram).
(15): Fuel manifolds. See diagram for directions.
And so on. Not only is this helpful to me, it's written so that anyone over eight years of age could reasonably find anything on the boat, and could, via the exploded diagrams, divine its purpose.
Why go to the trouble? Because I have a passagemaker being readied for offshore cruising for five years. Everything must "have its place" because it might need to be found in a hurry. Also, not every hidey-hole is equivalent in terms of access or safety: I'm not putting the 18-inch crescent wrench I use on the shaft coupler in an overhead bin, as it would snap the latch in a rollover and take off a head.
Here's an example of this idea in progress: my boat's electrical system. Only part of this in done, but as it's to scale, it will allow me to figure out wire runs, lengths required and locations of fuses, terminal strips and so on. It will also help me to trace problems, which is a real issue on boats with all their dark corners and damp recesses.