Chris Pratt and Katherine Schwarzenegger Tandem Paddling with Rob Lowe

Chris Pratt and Katherine Schwarzenegger are two peas on a board -- with Rob Lowe acting as the third wheel ... or paddle, in this case.

The newlyweds had a fun day in the sun and on the water with Rob -- Chris' former 'Parks and Rec' costar -- off the coast of Santa Barbara. All 3 got in some paddling -- Kat and Chris on a tandem kayak, and Rob was solo on a stand-up paddleboard.

The guys bared their torsos on the beach, and the Pawnee public employees were looking great. Kat looked hot too in a red bikini showing off her toned abs.

As for the actual paddling, it appears Chris might've been doing most of the heavy lifting. He's definitely built for the work, looking pretty yoked these days. Those L.A. gym sessions are seriously paying off. 'Guardians' season must be around the corner, huh?

If you didn't know ... Chris and Rob are super tight. They golfed together just hours before Chris met his bride at the altar, and they definitely bro'd out on the green.