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