If the boat's a-rockin', ya gotta come a-knockin' ... 'cause it might just be 50 Cent whipping the crowd into a frenzy onboard!

TMZ has obtained video of the rapper surprising a ship full of people Friday night ... after Flo Rida brought him out as a special guest during a performance at a party hosted by Houston Rockets governor Tilman Fertitta's.

Check out the video ... Flo gives his longtime friend a wonderful intro -- calling him one of the greatest rappers ever -- before 50 struts out and drives the crowd wild.

50 Cent starts with his song "P.I.M.P" before moving on to other classics like "Candy Shop" ... with Flo Rida hyping everyone up beside him.

At one point, we're told the stars sprayed a bunch of champagne onstage ... and, it wasn't the cheap stuff either -- they showered Le Chemin du Roi from 50's Sire Spirits on the stage.

The event 50 surprised fans at was Fertitta's Salute to San Luis in Galveston, Texas ... an annual Mardi Gras event. Fertitta was recently named the Ambassador to San Marino, a microstate near Italy, by President Donald Trump.

"Many Men" may wish death on 50 Cent ... but, many more clearly want to party with him.