Got a beater boat last week and am trying to get ready for the water.I read the other checklist and that helped.I'm trying to assemble spare parts to carry with me.

Trailer-
correct lug wrench(have spare tire and jack in the car)
2 spare bearings and grease
spare bulbs

Boat-
spare spark plugs and wrench
spray carb cleaner
extra drain plugs
spare prop and cotter/shear pin

what else?I usually carry duct tape/wire/toolbox/jumper cables and tow chain in the car,just looking for small items that I might need for simple on the road/water repairs.

rope/bungee cords and zip-strips might be good to throw in the toolbox.