It strikes me as odd that RB has admitted he woke in the middle of the night to find her gone, even though the next logical questions after that are why didn't you raise the alarm/alert the authorities/activate the MOB protocol/jump up and down screaming and shouting like a loon. If he has something to hide (even something small in the scheme of things, like being too drunk to bother, say) you might expect that he'd gloss over that and say he woke in the morning to find her missing, then contacted LE when he realised she'd left everything behind. It makes me think something happened in the small hours that he had a reasonable concern might have been seen or heard by witnesses, creating the need for a story of some kind of activity on the boat. As it is, a witness claims to have heard a scream at about 1am, which may or may not be related.
On balance, I feel she did make it back to the boat but what happened then is a mystery. I could get on board with the idea of a row and an assault, and her jumping overboard and swimming to shore to get away from him, but I can't believe she wouldn't have made contact by now if still alive. If not alive, why is there no body? I don't know that region. Is it plausible that a body would just get washed away? Everyone seems agreed the water is shallow and clear, yet nothing's been found. From a murderer's perspective, it would be a risky move to dispose of a body overboard, surely, but perhaps if you knew local LE was slack it might be a calculated one.
If I weren't a complete duffer about all things maritime, I think I would be taking a close look at the local tide timetables for that night and the following day.
JMO/speculation