Investigators have concluded MLB pitcher Jose Fernandez was driving his boat at the time it crashed in Miami ... killing everyone onboard ... and say he was drunk and high on cocaine at the time.

Fernandez -- along with Emilio Macias and Eduardo Rivero -- were killed on Sept. 25 when Jose's boat smashed into a jetty around 3 AM. 

The FL Fish and Wildlife Conservation Commission launched an investigation into the incident -- and Thursday they released their findings. 

Long story short ... Fernandez's blood alcohol level was almost twice the legal limit and his blood tested positive for traces of cocaine.

Officials say Jose and his crew were boozing at a local bar before the crash -- Jose bought 2 bottles of Don Julio tequila and 3 other drinks.

Several of Jose's friends were texting each other that night -- apparently concerned for Jose's well-being after he got into an argument with his pregnant girlfriend.

As for the crash, officials say they believe the boat was traveling at 66 mph in the pitch-black night -- in a very dangerous area. The report essentially says Jose was incredibly reckless.

The report also described the physical injuries all 3 men suffered and it's gruesome.

The families of the other victims have already filed lawsuits against Jose's estate for wrongful death.