There is also the question of the snorkling gear. We know he had it with him. We also know that she was staying with him. So, if he had all this gear with him, and brought it with him, she would have known, and she would be expecting him to go snorkling. The idea that they were just props to disguise some other preconcieved plan is ridiculous, since she would surely have wanted to know why he was bringing that stuff with him if they were just going for drinks.
In other words she would have been expecting to go snorkling. What would be the point of going to the snorkeling spot, not go snorkeling and say that you did? Especially when there is evidence that she was there, in the form of the towel and clothes. We know that the witness claimed the vehicle left and did not return, so those items must have been left there. Did she leave naked? I don't think so.
That leads me to believe that the witnesses saw them arrive sometime (being unremarkable, they wouldnt have noted the exact time), then later saw a vehicle leave (again, being unremarkable, no exact time). During that time RG and GG went snorkeling, she got swept out to sea by currents/drowned. Then he gets to land, wanders around a while looking for her, then gets in the car to try to get help. All of this we know has a hard time period constraint of less than 2 hours. Including travel times and fluffing around, time spent on the beach probably didn't exceed much more than an hour.
If the witnesses did in fact see anything, IMO they did not have a clear idea of times, and simply saw a vehicle arrive, then leave. Only it left somewhat later than what they think it did. Unless there was something particularly remarkable about the vehicle, a casual observer might remember it arriving and leaving, but it is highly unlikely that they would have an accurate idea of how long that took.