Dennis Quaid Looking Ripped with Young Girlfriend in Hawaii

Dennis Quaid's tapping into the fountain of youth, or at least a very youthFUL girlfriend ... and they hit the beach, flaunting ripped physiques -- and a nearly 40 year age difference.

Dennis and his grad student hottie, Laura Savoie, are enjoying an island getaway on Oahu ... and Thursday afternoon they did what any fit tourist does in Waikiki, rented some boards and paddled out into the Pacific.

The 65-years-young actor ditched his shirt as he carved up some waves, showing off his super cut bod. Laura, a total dime at 26, kept it sexy in a tiny white bikini.

After feeling the burn of a stand-up paddle board workout, Dennis and Laura headed back to land for a romantic stroll.

Fun on the water is kinda Dennis and Laura's thing ... as we reported back in August, the couple vacationed in Italy and went cruising around Lake Como.