No marrow isn't cucumber at all - wrong texture and too high a water content for a marrow.
Given that in the UK courgette (zucchini) are about 6-7 inches long and about 1-1.5 inches across, a marrow (or sometimes "vegetable marrow") is basically a very overgrown zucchini - about 15 inches long and about 6-8 across (i.e. the size that zucchini were when I was growing up in Oz).