the only real problem is figuring out how to purify the ocean water and remove all the salt.
1. Dig a hole in the sand by the shore until you hit water.
2. Put a bowl or other container in the middle of the hole.
3. Place plastic wrap or similar material over the entire hole sealed by sand over the edges.
4. Put a small object on top of the wrap to weigh it in a downward angle at the bowl.
5. The seawater evaporates, forming pure water on the plastic wrap.
6. The now fresh water then slides along the underside of the plastic until it reaches the middle, dropping into the bowl below.