I am a minimalist when it comes to tools l cary on board my tiller operated 16ft boat. For the engines l carry one spark plug for each, c/w spark plug tool that incorporates a screwdriver. I change each engines fuel filter at the beginning if the season, if they fill with water, I just drain them, I have never had one plug with dirt, gas stations filter their fuel before it gets pumped. I have extra fuses for both motors and extras for the electonics tie wrapped under the engine cover in plastic bags where they stay dry. I keep a stainless multi tool in the tackle box. My pliers and fishing knives do double duty. I run separate fuel tanks for each engine, that can be used on either should the need arise. I used to carry a spare prop, but no longer do, one experience changing a prop on the water was enough, I now rely on having an extra engine.
For the downriggers I carry a spare belt, as the cannonballs show up on the sonar, l do not carry a spare counter.
For the boat, everthing required by law, hand held VHF, 200ft of 3/8 braided rope, canned water, fero rod, waterproof matches, cheap cdn tyre tarp.
Please keep in mind that I do not fish off shore, or out of sight of land. I have never been stranded overnight because of a breakdown, but have had to spend the night ashore because of weather.