As passengers cross the gangway to board the Becky Thatcher, they are scooted off to the side and instructed to pose for a picture behind an old-school life preserver bearing the date of the blues cruise that's about to get underway. "Oh, puh-leeeze," their eyes seem to say. But by cruise's end, two-and-a-half hours later, damned if they don't end up buying the photo: They are stoked, having just disembarked from the best party in town. About once a month, from June to October, the Core of Discovery's Mississippi River blues cruises attract a wonderfully diverse crowd of locals, tourists, families, couples, the dapper and the casual, who are ready to bust some serious moves to St. Louis' best blues bands. As the boat chugs up and down the river, passing the illuminated Arch no less than three times, the music will inevitably slow to a pace that matches the plodding speed of the faux paddle wheeler, and when it does, it lulls swaying pairs into a peaceful groove. Even the two-left-footed will find that rhythm is easy when the Mississippi's their metronome. Grab your partner for the last cruise of the season, departing at 8:30 p.m. on Thursday, October 11. Tickets are $18 to $20.