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. Click 'SHOW MORE' for related content...

Drake and Serena: Does Mom Approve? https://youtu.be/5iWhymM6hrg
Serena Williams LOSES, Is Drake Bad Luck?!!! https://youtu.be/6HdTt3dhLjE
Drake Rock Hard... For Serena? https://youtu.be/SllNyl5A3Ko
Serena Williams Pre-Battle Dinner With Kim K! https://youtu.be/fADr6VcSqBI
Drake Hookin' Up With Serena Williams (PICS) https://youtu.be/0rpdqImyuDk
Serena Williams -- Leopard Swimsuit-Wearing Wedding Crasher https://youtu.be/BDfm40sBKM8


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