Option one is to take it to a local independent laptop repair specialist and get them to remove keyboard and make sure that the water has completely gone. Even a small amount of tap water will damage the circuitry of a keyboard almost immediately. They will have all the tools to do this and will quote you a price to replace it IF that`s the only issue.
Option 2 is to attempt this yourself for which you will need to find out the make and model and post back here the details. (Someone will help you locate the service manual with disassembly instructions and point you to a reliable source of a replacement keyboard.)
Of course you could just attach an external USB keyboard and see if the laptop works anyway but you might run the risk of further damage if the water hasn`t been cleared and has got as far as the motherboard. As it dries out it may damage further delicate circuitry.