I'm just musing here, but has anyone tried watercolour pencils with these colouring books (haven't used the books myself, although I have thought about it)? They are basically colouring pencils, and they can be used like that, but they are water soluble and you can intensify the colour by adding water to the tip, or you can apply water once it is on the paper to produce washes. The only problem I can foresee is the quality of the paper in these books may not lend itself to the application of moisture and felt tips are not water based.
The other thing I would say, and I have said this before on these painting threads, is don't let the fact that you can't draw stop you from having a go at painting. Try watercolour techniques - washes, dry brush, spattering, wax resist - and see where it takes you. You might surprise yourselves if you aren't restrained by drawing.