Why am I getting hum and lights dimming with a new electric tankless water heater?
18KW is a 75 amp load at 240volts , which is a HUGE addition in draw for a very small house. For comparison, if you replaced a tank type water heater they typically have 2 4,500 watt elements but only one is on at a time for a current draw of just under 20 amps.  You say you ran 8 gauge Romex, I assume that's 2 runs one for each 240volt breaker.  

LEDs esp. with dimmers can be very sensitive to voltage changes.  I have some in my home, on the same dimmer that dim at slightly different rates even though they are on the same circuit/dimmer.

Last, I agree with Mazura, your power drop from the POCO may have been adequate before, but now, with that much larger load, isn't anymore.