First off, maintain your motor religiously. If you neglect your equipment, your list of tools to get yourself back to the ramp will just keep growing and still never be adequate.
You have to also accept that you will never have the right tool for any given job. At best, you will just barely rig it together adequately enough to get back to the ramp. Many times, you will pole back.
These two things will prevent your tool selection from growing out of control.
I'd pack:
Pull-rope w/ for when your starter battery or starter or alternator dies (one came with the motor/don't lose it); I've had a starter fail AND then the pull-rope broke. I upgraded to Kevlar rope and also carry an extra.
Adjustable wrenches are for big bolts that are easy to get to. There are very few of those on a boat, so I'd leave the adjustable wrench on the truck. I find a ratcheting screwdriver with an assortment of bits to include Phillips, flat, Torx, hex and compatible 1/4" sockets (for God's sake, don't forget the 10-14 mm metric sockets) to be really handy for on the water purposes. Good needle nose pliers with integral wire cutters are more helpful on the water that regular pliers. In this capacity, I actual prefer a good multitool with carbide cutters because the cutters are great and you get a knife and file in the same package. You can perform miracles with just a ratcheting screwdriver and a multitool.
A small brass wire brush and some aerosol terminal cleaner are really handy for cleaning off built up corrosion, grime and grease so you can actually see what you are doing.
Don't forget the expendable materials. You need spare fuses, spare drain plug, spare key, duct tape, electrical tape, soft steel wire, JB Weld, zip ties, hose clamps and extras of any bolts and nuts that you are likely to drop overboard (jackplate bolts, battery terminal bolts, prop nut, etc).
Carry a spare prop and necessary tools IF you are likely to break it AND you can fix it on the water. Otherwise, don't bother. Many guys have slowly limped home with fractured props.
Never forget the TP, but get a waterproof container that will keep moisture and mice out of it.
Nate