burnt ground wire?

Hello All, newbie needs question answered. Have 2005 90hp on 22ft pontoon. Had boat winterized 2 yrs ago. Wanted to get her up and running and the following happened. Charged 2 batteries and connected them to battery switch. Turned battery switch to #1 bat. and what appears to be a small thin ground wire located on the front area of the lower unit began to glow red and burnt itself up. Can anyone advise as to what happened here and how big a job it will be fix same?

[boats.net]

Check all engine and battery grounds, probably have a loose wire or cable.

[reelnative]

check and make sure you dont have a batt hooked up backwards, pos to neg, ect