Oh, no. I see this as a real film, with a miraculous second vehicle, something along the lines of "My Cousin, Vinnie" somehow exhonerating the swimmer...
Mary Jo staged her own suicide as a murder: Grabbed the steering wheel and caused the car to go off the bridge and into the river, shoved the clueless (and totally innocent Teddy, who forgot that he'd been drinking and was only trying to do the good deed of giving her a ride home) out of the car, and then positioned herself so that her nose was facing that air pocket to make it appear that the spirit-lovin' one didn't try to rescue her.
P.S. Mary Jo's motive: She knew she didn't have life insurance that would pay in the event of suicide, and she needed a way to see that her parents got a new house. ;)