a spark isn't good, you may need to check your fuses after that.
I'd sell the boat.
A small spark is a draw. You have something on like the ff. A large spark and the wire gets hot, that a short.
another thing you can do to check for a draw is hook up the neg wire to the battery. take a test light of 12 volt meter and put the one end on the pos term of the battery. Take the other end of the test light or the other lead from the meter and touch one wire at a time. If the test light lights up you have a draw. Some electronics will light the light with a dim light. That is a keep alive memory, like a clock is a radio or radio station preset. If you have an engine with a computer on it, it will also have the memory.
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