I've wondered the same thing, whether there's a set that would be sort of self-contained and easy to store. But since I want a prop wrench in my kit, that won't work. I also wanted some other oddball stuff it there. So I bought a canvas bag (no, it won't float) and keep in it the following:
- Spare plug
- Socket set
- Wrenches, drivers, & pliers in a tool roll
- Prop wrench
- Voltage tester
- Dielectric grease
- A few spare bulbs but since most things in the boat are now LED I'll probably never need them
- One adjustable wrench kept on top for the drain plug but which could be used for lots of other things
- A small roll of duct tape
- An impeller kit - not that I'd try to replace it on the water but if I needed to do it during a trip I could - see other posts about Volvo parts availability in Missouri

- A couple of feet of neoprene brake hose - in case I need to do any repairs around the trailer lockout solenoid during a trip; could also be used as a siphon hose, etc.
- A ziplock bag with 5 pairs of nitrile gloves
Having said all that, I agree with Walt that there are very few things that both
could happen on the water and
could be fixed on the water.