How to de-seed a watermelon?

There is no particularly easy way, you have to manually remove the seeds as most of them are still connected to the fruit. The way to make it efficient is to slice it thinly, remove the seeds you can see on one side, flip and remove from the other side, then cut it up into smaller chunks after you de-seed it. I use a spoon to de-seen mine, but I hold it close to the tip of the spoon so I can be exact with it.

You can also cut it into chunks and then de-seed each chunk from all sides, this does work but I think it is less efficient because you spend a lot of time turning each piece.

There are several hits on google, and many videos illustrating how to de-seed a watermelon. Most have you cut the watermelon length-wise to expose the rows of seeds, which are generally in a circle down the center of the melon (imagining the pattern if you were to look through the end). When you cut lengthwise, you expose the rows of seeds. I prefer to remove both ends, and the rind first. Then, slice in length-wise wedges. You can often hold a wedge in both hands and gently break it along the line of seeds. You can also cut the seeds out fairly easily this way, and create seed free pieces closer to the peel. However, there is quite a bit of melon surrounding the seeds. That, you just have to work at.

The way I, and all my fellow country-men and women do it, works only on well ripened water melons.

Imagine the watermelon is the globe and the stem is the north pole. Start cutting it at about 75 degree latitude along longitude lines, keeping the knife as perpendicular to the surface as possible (towards the south pole that will not be possible any more). Don't make the cuts wider than a timezone hour. Generally, thinner is better, but be reasonable. After n cuts, you'll have n-1 slices of watermelon falling aside.

Depending how thin your slices are, you will end up with some slices having no seeds, and some slices having seeds only on the outside, with very rare slices with seeds inside.

The seeds will be parallel to the exterior surface. Now you need to make a new cut just above the seeds line, knocking a bit the seeds themselves, resulting in a mini core slice that is seedless and a larger seedfull slice. Since all the seeds are now exposed, the only thing left to do is to turn the slice with seeds down and the rind up and knock the outside of the rind a few times with the back of the knife. The gravity will do the rest and voila, seedless watermelon from seedfull watermelon. If the watermelon is not ripe, the seeds will be too well attached and only part of them will fall.

Now you can slice the rind away and cut everything in blocks if you wish. Repeat with each slice.