The 54-year-old "NFL on FOX" star hit the water for the weekend with a little boat trip in Florida with his former Dallas Cowboys coach Jimmy Johnson -- and showed everyone he's still carrying around a 6-pack under that shirt!

"What a weekend," Aikman posted on IG ... "Lots of laughs with great friends reminiscing our time together in Dallas and celebrating Jimmy Johnson."

Johnson was elected to the Pro Football Hall of Fame in early 2020 -- but the COVID pandemic delayed the induction ceremony.

Now, Jimmy is on track to get his bust in Canton in the Summer of 2021 -- and Troy is fired up for his longtime pal!

"We are all indebted to Coach for changing our lives. Next stop, Canton, Ohio! #hof #dallascowboys."

As for Aikman, the former QB was inducted into the HOF back in 2006.

Troy is a known health enthusiast -- he's always either working out in the gym, spinning or jogging around his neighborhood.

... and the hard work shows!!!