I am really surprised that no one has replied to your question yet! Shame on y'all!
I am cruising in 2 weeks for the first time and since I am a control-freak, I checked out the excursions that I was interested in extensively. In almost all cases you can find an independent excursion.
For my excursions, I even found out who Princess uses for their excursions (heavy research in some cases) and on 2 occassions I called them in Alaska to ask them questions. I also compared them to the independents, and called at least one or two of them. Almost all the Alaska excursions are available by an alternate (and sometimes the same) company as the ones Princess uses. The price that Princess charges is about 10-20% more than the independent, but I think that is worth the piece of mind (since I'm new) to know the ship will not leave without me.
I also found that the excursions offered thru the ship are not necessarily the same. For instance, my son wants to see the Saxman Village at Ketchikan. The excursion thru an independent is about $36, but $48 thru Princess. BUT the Princess excursion includes a native indian dance exhibit which involves the audience, and this is only available thru the ship. Made to order experience for a Cub Scout! I found other examples where there was a similar excursion, but not the same. The helicopter excursions did appear to be the same, however.