Very unlikely you would be able to be on the dock by 12:30. Even if your lucky enough to get on the first tender a bit of rough seas, bad weather, an issue departing the last port late, etc. Arrival isn't guaranteed at 12 by any means.
As far as getting back on time is less of an issue but if you miss the ship they won't wait. If you do an excursion through the cruise line they will wait for. My suggestion is to call and let them know your departure time, they do this everyday, and can tell you if your ok. Some gaurantee return by a certain time to meet the ship. If you go minimum I'd let your tour operator know when the ship departs so they can make sure you get moving on time.