I have a 86 venture bass boat with a few things on the back. It has a transducer, a water wheel for the speedo, and something else I'm trying to figure out exactly what it is. Let me know if you guys know what it is, thanks! 

Follow the tube up and see where it goes, looks to me it is some type of overflow for the carb, maybe it was leaking and the person put that so it would not pool in the engine area.  

It looks like a pickup for a speedometer to me. 

I thought it was something to do with the speedometer but I figured that the water wheel would go to that. I am not home at the moment to check where it goes but it does have me wondering. It isn't anything to do with the motor though. 

Pitot tube/pick up tube for the speedometer. Tubing should lead/goes to the back of the speedometer (in dash) gauge be my guess. The water wheel is the speedometer reading for your depth finder display.

speedo tube.................

1880's? 

 Speedometer pitot tube and trolling speed.

Tom