Kylie & Kendall Jenner in Bikinis Drunkenly Tossing Rocks Into Sea, on Video

Kylie and Kendall Jenner are having the time of their lives down in the tropics somewhere ... Kylie just posted video online of her and her sister drunkenly frolicking on the sand.

Kylie posted a video on her Instagram Thursday, instructing followers to take a vote in the comments ... about whether the chunk she's holding is "rock or coral." The two are super enthusiastic about digging up chunks of white debris out of the sugary sand, and returning them to the sea -- with names.

Check out the video ... the pair is lovingly curating each choice, carefully considering monikers, and shrieking with excitement.

An unnamed man and a couple women walking by on the sand offer their own hoots and hollers as encouragement, with one of the women holding up a cocktail to the camera -- good times all around.

It's unclear where the pair is performing their treasure hunt ... the only info is a caption -- "drunk beach walks with @kendalljenner 😂😂enjoy. @drinksprinter" ... shouting out a specific vodka soda drink.