I think there are a few problems with that version of events. First of all, I do not think 'walking' back the Edgarton was an option - that's why there was a ferry service. Also, the bridge they drove off of was on the opposite side of the island - it went to a spit of land east of the island, and did not connect to Edgarton at all. Also, The Swimmer did several obvious things to establish an alibi (complaining about the loud party, ordering room service), which indicate that he knew about the accident well before he read the morning paper.
I think the sequence of events is simple: 1)Go to party with young women 2)Drive to secluded beach to fool around with young woman 3)Drive drunk butt off bridge 4)Go tell lawyer friends and swim off the island 5)simultaneously hope nobody finds the car, build an alibi, and make a few dozen phone calls to friends, lawyers, and fixers 6)the next day, discover the jig is up and admit to driving the car 7)bribe judges and prosecutors