Jumper cables
Proper tow rope
Wrench for the pump inspection cover
Big ass screwdriver for rocks in pump
Spare ignition parts depending on you applications
Spark plugs
Basic hand tools but I've never used 'em.
25 yrs ago we broke a stock SBC rocker arm.  That boat still carries a spare to this day (same motor)

I carry a portable BBQ but that's just how I roll. 

aside from a basic tool box assembled from a list when i built the thing...

spare fuel filter
spare fuses

Yes i was about to say the same thing when i built my 460. also  bring some extra electircal wire, extra tubing, oil Duh, duck tape fixes everything " only if it becomes your last resort" o yes extra hose clamps, possibly a fuel pump i always keep one in my boat along with another coil, and a solenoid, i have duel batterys in my boat but i keep and extra one up in the front just in case,  and maybe a 30 pack in the cooler in case you cant fix it and are stuck and need some todo to pass the time 

A telescoping Magnet to retrieve the bolt you just dropped under your motor

^^^  Now that is a fine idea.  Don't know why I don't carry one anymore.  Used ta

And a roll of toilet paper... you never know ... hahahhaha

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I agree with the above stated items. This one SAVED MY ASS and I always carry it now. I carry a brand new bilge pump. I extend the wires to about 5 ft. and install battery clips, and I put 6' drain hose on it. When the real bilge pump quits, and you can't get to it, you can just drop the spare pump, hook it to the battery and pump out the water.

I also had a boat with an electric fuel tank selector switch that failed on me. So now I also carry a cheap electric fuel pump (just in case) with two long hoses and long wires with clips. It has really come in handy over the years. I have used it to help others more than I have used it myself.

And last but not least, I like to carry some Dynamat just in case I puncture a hole in the bottom. My friend actually did that, we sealed it up, and he ran it like that all season. If you work in the trucking industry, get some trailer tape....same thing but way cheaper.

An extra alternator belt