Another issue is that the party-goers cleaned up all evidence of drinking before the authorities arrived at the cottage. Not an empty bottle or beer can anywhere to be found.
I always thought that two cars made the right turn...both heading out to the ‘point’ for some beach rendezvous. Kennedy likely in the rear. The second car reacts after getting the destination and waiting twenty minutes, thinking something went wrong, and comes to find the car submerged and Kennedy climbing out of the water. They drove back to the cottage and calculated the mess involved, and then 2nd guy (with his girlfriend in tow) drove Ted to the point to swim across, and this guy and gal cleaned up the cottage and set the plan up for Ted to avoid any criticism.
It’s one of those events that you just don’t believe the way that the story is told...it doesn’t fit.
I will add this...if Ted and his associate had been smart (something rarely seen)....they would have pulled the dead Mary Jo out of the car, laid her down on the ground and made Ted appear to have tried saving her.