Greg,
You'll get a lot of opinions on that question - I'm regularly 40 miles or more from Home Port on both the Chesapeake and off Hatteras, so I'm a bit obsessive. Here is my list of what I carry on board my 25 ft C-Hawk cabin boat...
1. Spare prop, including castle nut, cotter pin and thrush washer in a zip lock bag taped to the prop.
2. Duct tape (Yes, I'm a Southerner, and duct tape can fix a lot of things!)
3. Set of metric sockets and drivers.
4. Set of metric wrenches
5. Set of metric Allen wrenches
6. Set of screw drivers
7. Spare kill switch yoke
8. Spark plug socket (or the little Honda tool kit that came with the engine in case I need to pull a plug) plus 2 or 3 spare spark plugs.
9. A piece of canvas that can be taped to the engine to catch any parts that get accidently dropped.
10. WD-40, small can of machine oil, and small container of marine grease.
11. Standard pliers, channel lock pliers, small and large vice grips.
Items 2-11 (except the piece of canvas) are stuffed into a single plastic tool box t which I fabricated a soft rubber seal around the lid rim. Inside the box are one or two rags soaked in WD-40 to prevent rust.
12. Quart of oil
13. Pint of hydraulic fluid
14. Manual water/bilge pump
15. Marine Tex, 2-ton epoxy, and a set of large C clamps.
Also, I have a separate sealed electrical box containing volt/ohm meter, fuses, jumper wires, small roll of #12 wire, small roll of #16 wire, butt connectors, various terminal connectors, electrical tape, shrink wrap, wire cutter, wire stripper, and butane fireplace lighter for using shrink wrap.
A few other things I keep on board that are useful - a bottle of hydrogen peroxide that is always within easy reach for cuts and scrapes and a box of band aides in addition to what is in my custom first aide kit.