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Boston drummer Sib Hashian's official cause of death was a heart attack ... Sib's son, Adam, tells TMZ.

We broke the story ... Sib had been playing a show with his five-member band, Dirty Water, aboard the Legends of Rock Cruise when he suddenly collapsed onstage.

Adam says the band was only 3 songs in and performing a cover of Boston's "Party" when Sib collapsed last month. He was pronounced dead shortly after midnight the next day.

Sib’s body remained on the boat till it docked two days later in Miami and he was eventually cremated.

We're told he had his affairs in order before his death since he had been diagnosed with kidney cancer at the end of 2016.

Sib was 67. RIP.

Another one?! RIP Sib.

Damn that is very sad. Rest In Peace!!