Most marquees are made of woven synthetic tent fabric. This is similar to the fabric used for window awnings, tent awnings and sun umbrellas. You can impregnate it with Sprayhood & Tent Protector.
However, some marquees are made of very cheap material. These are so loosely woven that it is difficult to make these types of tents watertight. All you can do is try to impregnate the tent cloth several times. That way, you seal the minuscule holes and increase the likelihood of making the tent watertight.