Wow. Hadn't considered that Ospreys would be this kind of problem. We have a bunch of seagulls and cormorants that love the small rock islands near our mooring. The boat yard was going to erect an osprey nesting platform to scare the others away. I'll let them know that they may just be shifting the problem to possibly something worse.
To protect our weather instruments on a commercial job we have in Kuwait, we get a small rod, and wrap it with masses of cable ties, pointing in slightly different directions. Simple, cheap, lightweight and effective.
There are several techniques that will work. I have found that a combination of a continuously barking dog and loud music interspersed with firearms discharge will clear way almost any annoying bUrd's. During times when loud music and firearms are not available it is often useful to allow numerous banging halyards. Best technique for use when moored in the slip.
I had one trying to build a nest on my masthead. I rigged up a length of 1" x 1" like a "pigstick" that yachties use to hoist their club burgey, attached some pink survey tape to it to flutter in the wind, and raise it up. The top extended beyond the masthead. That did the trick--he (or she) went over and built a nest on a neighbor's dock instead.