Serena Williams proved her ass is buoyant ... floating it down the San Marcos River in Texas this past weekend ... with the guy who played Oliver Queen's sidekick on "Arrow."
The tennis superstar strapped on her bathing suit and grabbed some inner tubes -- along with Colton Haynes ... who played Roy Harper (aka Arsenal) on the CW show.
The San Marcos River is basically a summer party spot -- where people grab beers and other alcoholic beverages and booze away as they float down the river.
No word if Serena drank during her river run -- but since she pressed pause on tennis for a while ... it's not like she's gotta be in playing shape any time soon.
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