It's Frank Muto firing the cannon! Or at least Master Billy says as much when he finds Capital Sam in the chaos. For a minute Sam is like "Ugh, how did I get dragged into this mess?" because Frank Muto and Sam don't have any particular quarrel.
Then the next part is a bit confusing. It's not Frank Muto firing the cannon. Someone named AL MACDONALD starts talking through a megaphone, telling everyone about a boat called the DREAM CATCHER which is available for rentals. Then someone named MISTER HORN takes the megaphone away from Al and talks about how the Dream Catcher is owned by Frank Muto, and Frank has loaned it to him (Horn) for the afternoon in order to track down the people who commandeered the Spire of Ice.
Basically he's saying he wants to see Capital Sam and Allison Hugo front and center, pronto. But not in so many words. And he's the one firing the cannon, OK? (I was wondering how well Frank Muto could really fire a cannon if he didn't have feet to stand on.)
Sam tells Billy to get back on board quietly* and have Allison prepare for departure. May is not really perturbed by any of this, she's got brass.
This story is actually getting pretty interesting, I wish the chapters were forthcoming a bit more fast and furiously.
*This part got LOLs out of me. There is something about secret disguises that is so funny. If you saw the movie Shanghai Knights then you know what I mean.