Obviously, this is a loaded question, and there could be a myriad of answers, but we feel that the number one issue, hence the number one thing to ensure, is that your batteries are always FULLY CHARGED, and you never allow your boat to run under low-voltage conditions.
There are so many sensors and parts in these boats that are sensitive to voltage. In the automotive world, where cars are typically driven every day, the battery stays fully charged at all times. Everything is so happy, always receiving a full dose of 12-14v.
However, in a boat, people are more apt to let their batteries deplete as the boat is stored, or run it all the way down jamming out to Lawrence Welk, they let their batteries get low. This can result in many maladies, from E-Reverse modules tripping its breaker, engine relays being ruined, among other bad things. Don’t do it.
Turn your main battery switch off when boat is not in use.
Install an on-board battery charger/maintainer like a Noco, and keep it plugged in. Keep your batteries fully charged at all times!
This alone would prevent a huge majority of the issues we see people having!