Update #2--I've had my water heater for 1.5 years. I've now got water leaking all over my basement floor. I will bet you a dollar that they will wiggle their way out of any warranty due to my failure to purchase an expansion tank. Which in 30 years of having properties, I've never had an expansion tank and never needed one in the past.
Which is what they wanted me to do when I bought the water tank. And they made it a point telling me both on the phone and in person that if anything went wrong, it wouldn't be covered under warranty. I don't even think I'm going to bother calling them, because they'll certainly charge me a service call on top of whatever parts/labor they won't pay for.
A water heat should last more than 18 months without leaking all over the place. I'm pretty disgusted.