I don't think it was necessarily Tatoo, and I don't think it is necessarily Croes that points to it, though he's the link to the sea. But if she was disposed of (dead or alive) offshore, which seems likely, and there's no trace of her onshore, then it's likely she went onto a boat willingly, and anybody would like to go for a late night boat ride on a beautiful night their last night in paradise. Even if boat riding with strangers is stupid.