The biggest issues would arise from a homeowners insurance claim due to damage from the water heater to your home in the future. If your homeowners insurance finds that you did not obtain a permit, then they may not cover the damages. Also it could come up while selling your home, or if you are reported by a neighbor. Most likely a fine would be levied or you would be asked to have a licensed plumber pull a permit and either redo or certify the installation.