battery and grounding

I was cleaning my posts and the socket touched a metal side while on connected to the positive posts and it arced. Is this normal? The negative terminal was connected as well but didn't touch that.

Quite probably. If you follow your negative battery lead, you'll most likely discover that it's connected in some fashion to the boat hull. It might go to your motor's electric starter, but that, too, will ground the hull through the engine mounting.
Anytime you connect the positive to the negative, (whether accidental or on purpose), without something utilizing that power, (like a light or starter), you get uncontrolled amp draw through the connection, thus arcing and sparks. It's not something you want to do, or keep doing.

Roger

You don't want the hull to be part of the electrical system.

You should not use the hull for grounding, but it is going to be grounded from being in contact with the outboard, nothing you can do about it.

That is why you disconnect the negative cable on the battery before disconnecting the positive cable(on a negative ground system). You do just the opposite routine on a positive ground system. Really bad things can happen by shorting the battery.

Thanks guys. I came to realize after that I should just disconnect the negative terminal first.

Appreciate the feedback.