Pirates in California?

That's right.

To me, this looks like an empty mineral bath at a hot spring, but it is a wine vat at Mission San Juan Capistrano in California.

First, the mission grapes were dumped into a similar vat in a room next to this one (the people at the mission think this vat was originally indoors too, but the walls fell down). Then people stomped on the grapes, and the juice flowed through a hole in the bottom of the vat out to this one, where it fermented.  I'm assuming that the wine was then put into casks for ageing. 

It is these casks that Magdalena is loading into the wine cellar at the beginning of the story. (Yes, you would have to be very strong to do this.  That's why Magdalena made a good pirate).