Dixie

Hi, I have a 2007 speedster 150. Last weekend put boat in water and went to start and no chirps at all. Tried several times. Found a pencil eraser and cleaned contacts on key and post and it started. About 5 minutes at 45/50mph it sounds long beep 5 seconds and stopped. A minute later it sounded again. Then again. Turned boat off and noticed depth finder was not working restarted twice and then it worked. As I was shutting down SENSOR was written on dash. Cleaned contacts again on key and post and restarted back to marina. No problem no alarm no sensor warning. What do you think? Bad post? Bad key? Or a sensor? Any suggestions will be appreciated! Thanks