It wasn't in his best interest to let her drown, but Teddy was a married man leaving a party of several young single gals and a few older married men. I believe that Teddy drove off the bridge on the way to a secluded spot to put the Kennedy moves on Mary Jo. I also believe that Teddy was very drunk, unable in courage, mental or physical capacity to try and save her, so he deserted her, walking past at least four homes where he could have summoned help, and instead spent 9(?) hours sobering up and marshaling as much support as his political office and Joe Kennedy's money could buy.
I posted from memory, no doubt there are web sites that have the facts laid out more precisely.
You summed it up nicely. Also, his first thought was for his OWN political career. While it would never do to have a young woman drown in his car, worse, thought he, would be for him to appear INTOXICATED when he went for public help for her.
So he swam over to the mainland, changed his clothes, appeared in the hotel sauntering around for an alibi late that night, and then early the next morning he went to enjoy an al fresco breakfast with some friends on their terrace, seemingly perfectly fine.
He wanted to keep his political status intact more than he wanted her alive. I think he thought he could literally get away with it.
Um... He did.