Tonga questions

Where are the nicest beaches in Tonga (in Ha'apai??) and how can I get there? only with chatman's pacific or take a ferry? and is it possible to do day tour whale watching or swimming trips? I've just found very expensive days trips. I'm going to be in tonga end of july / early august (during the coronation) maybe do you have any advice? where can I attend the celebration?

thanks
katinka