If I wear my watch too tight, can it restrict bloodflow?

Sure you can... Tight watch, ruberband/wrist-sport bands or anything too tight around the wrist can restrict blood flow--first the veins. If the veins on back of hands bulge out/larger with your tight watch, then it is restricting veinous blood flow. Further tightening can reduce arterial flow but this will be quite uncomfortable/painful if prolonged. So, loosen it up a bit. Good luck.