How to grow Watermelon?

Seeds

Seed Viability: 2 - 4 years

Germination Percentage: 80.0

Watermelons are cross-pollinated by insects so you can either grow only one variety at a time, isolate by a 1/2 mile, cage them or hand pollinate.

Hand pollinating Watermelons isn't difficult, though the flowers are smaller than other Cucurbits. The flowers are most receptive to pollination before any fruit start growing. Once there are fruit maturing on the plant they become less inclined to produce more by pollinating successfully.

You need to prepare to pollinate the flowers the night before, by finding some male and female flowers (the female has a tiny "Melon" behind the flower) that are about to open the following day. Tape them shut with a little piece of masking tape (don't tape it too well or it will be hard to remove without damaging the flower). This prevents them from opening and being pollinated before you get to them.

The next day pick a male flower and remove its tape and petals. Then carefully open a female flower (from a different plant), being very careful to not damage the petals. Insert the male flower into a female flower and brush the pollen from the anthers on to stigma. Finally tape the female flower closed again. The petals will soon fall off and if pollination was successful the tiny fruit will begin to grow.

If you are adventurous, you could collect the seed from any ripe fruit as you eat it (so long as it isn't an f1 hybrid) and see what grows. In the case of Melons, I would avoid hybrids for this reason, it seems a shame not to be able to use some of those seeds.