Seeds from an immature watermelon would be immature too.

Have you ever watched peas grow--being a newbie probably not so I'll tell you.
They start out with just the pod and a tiny, tiny nub where the pea will grow. That little nub grows into an edible pea but it is still not a viable seed. The pod has to get brown and the peas get old before they are ready to grow into a plant.

It is the same with any plant. The seed has to be very mature before it is ready to grow. With melons and those plants that grow their seeds inside, it has to be very ripe.

As to saving seeds at all--if you are going to go that route you should use heritage varieties. Hybrid plants seldom come true to form--grow a plant that is like the plant you took the seed from. Heritage varieties do because they are open pollinated varieties.

Good luck and welcome to the world of gardening

Thanks for the information, I didn't get a full grown melon and these seeds were real expensive.