The dealer made a serious mistake in replacing a 10 ampere fuse with a 15 ampere fuse. There is a reason why the fuse blew and since you are under warranty, it is the dealer’s responsibility to find out why.
The wiring to the windshield washer pump is designed to carry 10 amperes. The windshield washer pump, I am certain, draws much less than 10 amperes. When the dealer overfused the circuit, he was risking overheating these wires. The problem could be an intermittent short circuit someplace in the wiring or a pump motor that is defective and draws too much current. One other possibility is that there is another accessory on this circuit that is defective and is pulling too much power. Don’t replace a fuse with one that has a higher amperage rating than is called for.