If you want to start adding watercolour to any line work you are doing in your sketchbook, then you will need one with a decent quality paper. I enjoy using the “little black book” now branded by moleskine favoured by artists like Henri Matisse and Vincent Van Gogh.
Sketchbook study of Winter Tree
You can get one with a stiff cartridge paper which is slightly tinted that can take watercolour without buckling and they (moleskine) now do a sketchbook with watercolour paper in. The study above of a tree in winter was drawn on the spot whilst I was out walking my German Shepherd dog using a pen with waterproof ink. The colour was added from memory in the studio. This may well end up as a larger watercolor.