You could try drawing a very faint pencil grid on the watercolour paper. As you are drawing an isometric map the grid lines would allow you to draw straight on to the paper. Alternatively, if you didn't want to draw it directly you could also grid your original (or do the grid on trace so you don't ruin the source map) and then transpose the points (just google 'Drawing with a grid' for demo videos). As long as your grid lines are light enough, the watercolour should hide them. Alternatively use water soluble pencils, so the lines blend into the paint.