Suddenly, Melinda appeared on Robert's yatch. It wasn't his house nor an hotel where she can burst into anytime. The yatch was in the middle of an ocean or probably a sea. They should at least point out how she got on the yatch.
I don't really understand here too but the movie has made us understand that Melinda was crazy and can go any length. She will do things without thinking about the consequences.
Do you remember when she went to pour acid on that wedding dress?
Yes, she's that crazy. But, I was more concerned about knowing how she got there without anybody finding out.
Not even the watchmen Robert had on the Yatch.