Part of me wants to launch a legal battle to get the stream declared navigable, but I understand that this would serve as an invitation to everyone else to come ruin our fishing hole with overfishing and littering. The other part wants to explain to the golf course owner the risk he runs by forcing our hand to get it legally declared navigable. Step one - how would we begin the legal process? Step two - if we went with the latter (explaining options to course owner) how would we prevent coming across as extortionists?

I doubt they would think you are an extortionist , They will think you are a dumbass. You would need a funny thing called standing to open any sort of litigation (which you do not have) you would need a lawyer (which you do not have) or money to hire one .. And I do not know about where you are from but around here when people are spouting nonsense on the internet at 1130 on a work night , these are the people who do not have the money to do any of this .

On the other hand maybe the owner will get a good laugh out of you ... I did ...