I have been making a lot of journals lately what I have been doing and since you want to use watercolor paper might be perfect for you. I get a pad of watercolor paper and make sure the back board is a thick hard board. Then I carefully tear out the pages and fold all of them in half. I use the back of the pad ( the hard back part) and cut that in half this is what I use as the cover and decorate how ever I want to . I then sew the pages to the cover using lots of various stitching. Coptic is easy to learn. Or if you want to cut the pages japanese stab binding.
I just find that by doing it that way I don't have to buy separate book board.
I have also bought presentation board which is available at my local craft store. It is with all the art paper (its much thicker then poster board but has no foam in the middle. )