Justin Verlander -- Plays Pirate with Kate Upton's Booty ... Witnesses Celebrate!

Justin Verlander and Kate Upton are back together, and no one's happier than a boat load of teenage boys in Tampa.

Sure, sources tell us the boys in this video shot today were definitely cheering more for getting a peek of Kate in-person (primo material for any hormone-filled boy) -- and probably never noticed the Cy Young winner.

But still ... the Detroit Tigers ace and the supermodel were spotted all over each other -- partying on a boat for Tampa's annual pirate-themed Gasparilla festival.

Justin sported the traditional buccaneer eye patch as a costume. Kate's costume -- just looking hot.