wait youre saying its possible now?
Yes and no.
1. With the blocks we have er hab cold everything. Even ships. But they are not able to move
2. Some small boats are already in the game.
3. What you asked for is technically impossible yet for today's hardware. Imagine a huge ship, made from thousands of blocks, and the game would have to calculate each voxel and its position again and again while you steer it across the ocean. not to mention the information stored for each block how is connected to other pieces, the size, texture, etc.
That's an insane amount of calculation
But there is hope and the dev is working on that.
Premade modules that you can individually put together to create your own ship or boat.
It's in the making 