They are designed to be used with a variety of mediums listed on the wrap: charcoal, chalk, graphite, pencil, pastel, oil pastel, wax crayon, red chalk, acrylic, collage, oil, marker, spray, tempera... Everything except ink, my prefered medium. As I am part way through the A5 Seawhite sketchbook, I decide I would test drive the soft cover A4, with holding elastic. It has a handy niche on the edge keeping it both in place and away from the pages - neat!
Getting my map of locations, & sticker for my sketchbook, I drew the church clock and spire of St. Magnus the Martyr listening to the chimes of the bells. I added the watercolour, which the page handled well I am very lazy, mixing colours direct on the drawing with quite a bit of water. It crinkled a little, but not much at all.
I had a lovely day, thanks to Pete's planning, and look forward to using the sketchbook more as so far I think it's really good. They certainly should add ink to the list of mediums that it's good for. Now, I must try something other than watercolour or ink for a thorough test.