Here is one that is often overlooked. Many times and on many different types of equipment improper grounding can be the culprit that causes equipment to malfunction or not operate at all. I have seen this on boats as well as motorcycles and other equipment. I have actually taken the factory ground wire and replaced it with welding lead wire with soldered on Gold anodized connectors to make the ground have the continuity it needs.
Do not be afraid to double up on the grounding leads. Doesn't hurt a thing. On boats many of the long OEM wires are so small a gage/type they will cause a voltage drop by the time it reaches the device it powers. A good example is aerator and bilge pumps. I have seen a voltage drop down to only 4 or 5 volts at the pump connection. Ever wonder why these little motors burn out so often other than getting full of zebra mussel shells and locking up? Low voltage caused from not being grounded properly affects nearly everything on your boat. Look at the ground wire. Is it smaller that the positive? Most of the time it is and then you have everything in the boat running off of this insufficiently sized wire and with a tiny bit of corrosion over time the voltage drop gets worse.
Most ECM or PCMs (electronic control module or power control module)will require a minimum voltage to operate properly and provide the proper spark. Some of the ECMs require almost 14 volts to power up correctly. If you have a weak ground then you may not be getting but 10 0r 11 volts to it if that.
Is you engine hard to start? are your plugs fouling prematurely? Do you pump motors seem lazy? Try adding another ground wire.

Livewell timers is another one I forgot to mention. Especially the older shotgun shell timers. Timers work off of heat and resistance. Once there is insufficient voltage the timer may not cycle like it should and may fail prematurely. Instead of 30 second run times you may see more like one minute or more with longer off times. Bait dies when aerator don't work correctly.