When Is A Boat A Ship

While going through some of my photos that I took the other day on my walk around Stanley Park’s Seawall I asked myself this very question: When is a boat a ship?

Well according to Answers.com:

-          A vessel of considerable size for deep-water navigation

-          A sailing vessel having three or more square-rigged masts.

Now if you look at the picture I took below (I had to do some cropping to try and focus on the yacht itself. Would you consider this a boat or a ship (excluding yacht of course)?

What would you consider a boat then? A small Bayliner that you would take out on the lake I would consider a boat, but then at what size would you start calling it a ship?