I haven't used too many watercolor journals, but recently tried the Pentalic Nature Sketch book. It's 25% cotton, 130lb/212gsm. Perfect for mixed media with a watercolor base. Also, quite affordable ~$14/50 sheets/100 pages.
I also recently used a large, single sheet of arches cold press (100% cotton/140lb/300gsm) to make an accordion journal. It has 30 small squares to work on, so a strangely affordable "sketchbook" for Arches.