An unknown wire

Are you saying you pulled the engine and forgot to undo the big red lead going to the starter? Or this was already disconnected and you just noticed it now that the engine is out?

This is the only pic I could find one one of my starters. In the bottom right of the pic, there's a giant red lead going to the starter, then another smaller one. From the looks of it, that's 8 or 10ga but can't be sure.

 

There could be 2 big red wires in that area... one to power the starter, and one to power the DC breaker panel. The one for the DC panel will usually have an inline fuse... like 50A.

 

That's the wire that goes to the 50 amp breaker and should be connected to the 90 amp fuse at the starter.

 

There may or may not be the 90A fuse at the starter for this wire.
The fuse was added by Mercruiser on the newer models, may not be on a 2005

 

well...
Keep in mind the reason Mercruiser added that 90A fuse at the starter is because the 8AWG was unprotected and cant handle more than 80-90 amp. If it shorts (chaffing) between the starter and the 50A breaker or the alternator it will smoke. It was considered a fire risk and they added the circuit protection