Looks to me like something to hook a bike to for indoor exercise (turning a road bike into an exercise bike).

It does bolt to a boat, but that is not the purpose.

no, this object wouldn't be found on an outboard.

Yes, both very true.

Ok maybe the fluid isn't water but air and you bolt it to a boat as a wing for stability or it can be swung flat as a brake?

Getting colder

This is gonna end up in my claiming that I got it already when I said it was a waterfoil and you were too technical when you didn't acknowledge my victory

I conceded it attaches to the boat. It goes into the water like you said but it is not a foil for balancing or braking. The use of the object is the most important thing.

Right now, the equivalent is me showing you a bracelet and you telling me it is handcuffs. If that's a victory, then you are victorious.