Here is the bottom line on the Chappaquiddick case.
The events in the early morning of July 19, 1969 are bookended by two reliable timepoints. One is the sighting of Kennedy's car, containing one male and 1 or 2 female occupants, on Cemetery Lane on Chappaquiddick Island, 12:45 a.m., by Deputy Sheriff Huck Look. And it was Kennedy's car; the chances of another late model dark colored Olds with the license plate H7...7 being on the island at the same time are minuscule, especially since the operators of the only ferry to the island never saw another such car. When Look approached, the car sped off along Dyke Road, a sand track: clear evidence something was wrong. Kennedy knew Dyke Road; he had driven it twice earlier that day.
The second time point is Kennedy, in dry clothes, asking the owner of the Inn where we was staying, in Edgartown, across the channel, for the time, at 2:24 a.m..
In the interim, he had at the minimum, to travel the length of Dyke Road (0.8 miles; maybe 5 minutes, given the state of the road), crash the car with Mary Jo Kopechne inside into Poucha Pond (5 minutes, perhaps; perhaps much longer), walk the 1.6 miles back to the Lawrence Cottage (30 minutes at a very brisk pace, especially given much of the journey was on sand), roust out and provide some sort of explanations or instructions to Markham and Gargan, his confederates (maybe 10 minutes), drive the three miles to the Chappaquiddick ferry dock (maybe 5 minutes), find and unmoor a boat and take it across the channel, then moor it (perhaps 20 minutes) and then get up to his hotel room (maybe 5 minutes). That adds up to 80 minutes. He could have done all of it, at breakneck speed, by maybe 2:05 a.m.. And this coincides with a report of a boat carrying three men tying up in Edgartown harbor at exactly that time.
Could he have done it all in wet clothes? Maybe. One witness (LaRosa), who encountered Kennedy on his return to the cottage, went out of his way to emphasize he couldn't see what Kennedy was wearing. Wet clothes would have impeded his walk, somewhat. And he would have had to change into dry clothes at the hotel, before coming down to ask the time, but he had the ~ 10 minutes he needed to do that. Asking the time is of course a time-honored way of establishing an alibi.
Could he have escaped from the Olds, a big man, impeded by the steering wheel, wearing a back brace, upside down and certainly submerged head and chest in seawater, through the mostly open, driver's-side window? I don't buy it, but it might be within the bounds of possibility.
Could he have dived seven times to try to save her, and then lain exhausted for an indefinite time at the roadside to recover from his exhaustion, as he claimed? No. There isn't time.
Could he have returned, as he claimed, to the site with Markham and Gargan, to try to rescue her? No, there isn't time.
Could he, as he claimed, have swum Edgartown channel back to his hotel? No. First, there isn't time. Second, it's doubtful anyway. At that time of night, the tide would have been running at its strongest into Katama Bay. Anyone diving off the Chappaquiddick dock would have been swept way downstream, and had to walk back to Edgartown. The diver who recovered Kopechne's body tried it at the same stage of the tide later that afternoon, along with two strong swimmers, and concluded he could only have done it by starting far upstream of the dock, or ending far downstream of Edgartown.
Conclusion. Kennedy lied about his story from start to finish. He might have been in the car with Kopechne when it went off the bridge, but merely assuming that makes it a little harder to fit the timeline. And no one ever saw him wet, except his confederates. It is more likely he never got wet at all. The most probable scenario is that he guided the car, idling in low gear, off the bridge, steering through the open drivers' side window, and then hoped to get Gargan to be the fall guy for the 'accident', with him with at least a partial alibi in Edgartown. Gargan wouldn't play ball.
By the way, Farrar, the diver, estimated that Kopechne survived up to two hours in an air pocket at the floor of the inverted Olds. He said her hands were positioned in such a way as if she was trying to keep herself erect. That makes Kennedy's actions, in failing to secure easily accessible help after he put her in this position, at the very least manslaughter. If he'd wanted her rescued, she could have been rescued.
Of course, if he wasn't in the car, which I think is better supported by the data, it was premeditated murder.