Exactly where? If it is on driver side, it is possible it is only a radiator/heater hose. If it on passenger side, it can be hose or quite possibly water pump.
Yes, very serious. If you run out of coolant while driving, you may overheat the motor. Warped head or worse, cracked block. And no, do not add water if you overheat. You may instantly warp your cylinder head. Wait until it cools down.
Depending on where you live, it may run you in the neighborhood of 300 - 400. Change your timing belt while you are at it.
There is no quick fix if it is the water pump. I guess you can keep adding coolant (not water), although it is not good for the environment and possibly your wallet.