Robert De Niro Celebrates 81st Birthday with Daring Yacht Plunge

Robert De Niro 's still looking like a raging bull as he celebrates another birthday ... taking a wild leap off a yacht while partying with his family!

His daughter Drena De Niro was recording when her pop took the dive for his 81st birthday, and let's just say his form was not Olympic in its nature.

Bobby D looked like he was lining up a standing pencil dive off the side of the boat, but midway he rolls over onto his side ... and ends up doing a full body flop into the blue waters. Drena, his oldest child, was immediately shocked and concerned.

De Niro yelled back, "I'm okay," as he was treading water -- and Drena told her father, "You're crazy!" She asked some other people on the boat how high the fall was, and they said 30 feet!

Damn impressive ... especially at 81.

It's unclear where exactly the De Niro fam is celebrating, but it looks warm and beautiful. Drena also posted several throwback pics of the Hollywood icon ... including some with her late son Leandro , who died last summer from an accidental fentanyl overdose .

ROBERT DE NIRO THROUGH THE YEARS

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After a rough year for his family, it's great to see Bobby D kicking back and enjoying life to its fullest -- but, diving might not be his thing.