Bobbi Kristina -- History of Falling Asleep Wasted in Bathtubs

Bobbi Kristina got extremely drunk the night before her mom died, fell asleep in a bathtub at the Beverly Hilton Hotel and flooded the room ... and it could be critical evidence in the homicide investigation.

Law enforcement sources tell TMZ, Bobbi was partying with Whitney Houston at the Beverly Hilton bar and got drunk and unruly. Bobbi Kristina went up to her room, drew a bath and fell asleep in the tub ... intoxicated. By the time she woke up there was water everywhere.

Sources tell TMZ, attorney Jose Baez, who reps Bobbi's BF Nick Gordon, knows about the incident and will use it to convince prosecutors Nick did nothing wrong ... that she simply took too many drugs and did the same thing she did at the hotel with fatal results.

We've also learned the night Whitney Houston was found dead in her tub, Bobbi Kristina was rushed to the hospital for acute alcohol poisoning. Whitney's PR team said it was for exhaustion, but law enforcement tells us it was excessive alcohol consumption.

It's relevant especially in light of the Medical Examiner's preliminary autopsy report, which reveals no obvious evidence of foul play.