In general, most 3D games use something called coordinates (XYZ values) that tell the game where your player is located on the map. In muck, it does the same for enemies, placing blocks, where the ship is, and where your player is located.

Using some mods, we can place a block in the shape of a square around the entire map. For this example I used fence posts. Then we simply count one side of the square and we can get the total area of the square. This is the total XYZ values within the map.

Now that we know the maps total area, we need to find only the land mass. To do this I needed to separate the land pixels and the water pixels. Simple paint editor did the trick.

Now with some mods, I can count the total land pixels and the total water pixels. With this number we can grab the percentage of the land mass. Below is an image of the same map from above.



The results of the black and white image are:

And we can use this as a ratio to find that our island has:

As we did with finding the maps total area, I did the same with the boat.
Boat's Total Area: 5361 Blocks

To calculate the chance of spawning on a boat, we simply take the boat's total area (5361 Blocks), and divide it by the map's total land mass (2358122 Blocks). This comes out to a 0.0022% Chance!

Using some mods, I created a way to automatically generate a world, check if you are standing on a boat (Record seed if true), and repeat. Below is a list of ship seeds that were found from searching through 100,000 randomly generated seeds.