Recessed Ceiling Kitchen Lights Dimming when turn sink on!!

So I just noticed the weirdest thing. In my kitchen I have 9 5" cans that I supply with incandescent 40 watt bulbs. Ever since I moved in I have had trouble with all of the bulbs burning out prematurely in this area only. I checked the neutrals and everything was fine. The lights are on a lutron dimmer switch on the wall.

Now here's the weird part, every time I turn on my kitchen sink faucet on, the bulbs in the ceiling will slightly dim. When I turn the sink off the bulbs return to their normal state. It's not a huge signficant change in brightness but enough for me to notice. How is this possible as a sink doesn't create any electric load. Keep in mind I am not using my garbage disposal at this time. Any ideas?

 

City water or pump? Electric water heater or gas water heater.

 

City water, although you're right I do have a tankless electric water heater in the attic. It must be that. It's on its own circuit and definitely not the lighting circuit though

 

But it may pull 80 amps on both legs of the 240 volts going to the house and the lights run off of one of those legs and neutral. Try just the cold water. Does the on demand water heater use two 60 amp circuits as I speculated? What is the size of your main breaker? Do you have electric heat?