Can I mix watermelon in with my batteries bake it
Very difficult to bake with watermelon, as it's almost all water.
If you want fruit flavor in baking without a lot of liquid, use a fruit juice concentrate or syrup (and adjust the sugar accordingly).
So be better just to puree the watermelon and use that?
Have a look at this selection of recipes for watermelon concentrates or syrups and see if one or more gives you what you want in terms of taste or texture.