Oooh. BIG topic. I'll try to add some food for thought:

1. How many people will be on the boat? This will determine size.
2. Where do you plan to go with the boat? If offshore, it needs to be seaworthy enough to handle anything the ocean's going to throw at it.
3. How long do you expect to be out there? Remember that on the ocean there's water water everywhere and not a drop to drink. You'll either have to carry your supply or have a desalinator. And you'll have to stock food for the same time period. Again, affects size.
4. What part of the ocean? North Atlantic? South Pacific? Consider weather patterns and temperatures.
5. How will the boat be powered? Sail? Diesel? Gasoline? Oars? You'd hate to run out of fuel out there in blue water.

For ocean travel you'd need tools and spare parts to be able to repair any damage that may happen. The ocean is an extremely hostile environment. If your car breaks down you can walk. If the boat breaks down ..........