Am I obligated to pay for the repair cost of the computer, knowing that I denied the cause of the malfunction related to water so that it would be repaired for free?

You are obligated to inform them that water droplets fell on the device, as this could be a cause for increasing its malfunctions. You may also inform them of any previous malfunction it experienced. You must repent to Allah and return to them and be frank with them. If they forgive you, then that is good. If they do not forgive you, you are obligated to pay the cost of repairing the damage caused by the water droplets.