I find this to be an original idea. Never thought of Celestia as a swashbuckling pirate. She would definitely be one of the good ones, fighting evil and never killing anypony if she can avoid it. The art is very good and I see nothing in particular to criticize. I can imagine the Princess of the Seas sailing her mighty galleon (hopefully not called the Celeste, lest the ship and crew find themselves cursed to vanish) fighting some Diamond Dog marauders on the waves, likely alongside her first mate and beloved sister Luna. Naturally, no crew would be complete without the Mane Six aboard and Derpy manning (ponying?) the crow's nest.