The only thing needed to open up the Holly option on the yacht is to move around the "Holly-roam" screen with her until you see no more dialogues pop up as you move around. There is a total of 9 screens with 0 to 3 dialogues each, depending on your previous choices.
If you want to aim for the Holly ending (not a real ending, but a lightweight one), you will have to check Holly's contact info on your phone after the yacht and the "love meter" has to be completely full by then.