The tendering procedure varies throughout the day.
Generally, early in the morning, there is a crowd waiting to tender to the port.
DCL has a system where you will be provided with tickets and then you can go into the Walt Disney Theater to wait for your tender boat. Those with DCL booked excursions leaving early in the morning are given preference to go ashore.
As the morning progresses the crowds die down and DCL stops using the above method. Instead they just direct you to one of the Tendering Lobbies and you wait in line for the next tender.
Depending on the crowds, you may have to obtain a ticket and then wait in the theater for the tender or you may just be directed to the lobby.
Even in the event that you have to wait in the theater you shouldn't have to wait for long. DCL does a great job of ensuring that several tenders are running at the same time so there is almost always one at the dock on GC, one being loaded at the Magic, and one in transit between the two points.
BTW, we've done the 11:15 Captain Marvin's and loved it. I think we had to wait in the theater but for probably no more than 10 minutes.
Enjoy your cruise!!!
