We have taken multiple cruises on Windstar ships and the tenders are boats, not inflatable zodiacs. The only time the zodiacs were utilized were for the scuba divers to get access to their dive spots.
With that said, last December we were on the Windsurf and due to high swells, it was tricky getting on and off the tender. Plus it was a little precarious to get to the tender. You have to walk down a narrow and somewhat steep set of steps to a platform where the tender is waiting. This may prove difficult after your surgery.