RCCL Excursion Question

[CeCe]

I hope I can state this clearly and briefly. Our RCCL Alaska Itinerary gives us the docking and leaving time. Our ship is offering an excursion (leaving 2 different times) we are interested in. We are confused because the 1st leaves 30 minutes BEFORE we dock and the 2nd one will conclude when we are to leave! How can I select the excursion? If this was an independent tour I couldn't/wouldn't book because we miss our ship!!! Yes, I have emailed RCCL but it has been 2 weeks and no reply. Any insight to this?