I’m also in a cold climate (MN), but I have a gas tank water heater. My first impression is that the estimates aren’t unfathomable, but suggestive that your household goes through a lot of hot water. How many occupants? Are you always doing laundry with hot water? Showers have lo-flow faucet heads?
Using this calculator, I typed in a # of gallons until I got close to the annual projection, it suggests that you’re consuming about 164 gallons of hot water a day. The costs, of course are harder to comment on since we all have different rates. But I’d suggest using this calculator a bit and seeing if you can get comparable numbers and then think about hot water usage and determine if it all seems reasonable.
Our forced-air furnace is our only other gas appliance, so I can look at my summer gas consumption (only about 8 therms/month) and get a good idea from the calculator about what our hot water usage is (about 25/gallons per day). 25 gallons per day may be low to some, but for our household I think it sounds pretty reasonable. We only wash laundry on cold, and so 25 gallons is about what we would use for just the two adults in our house.
Based on your description, I’m guessing this is your model: https://www.rheem.com/product/marathon-100-gallon-electric-grid-enabled-water-heater-with-limited-lifetime-warranty-mrg105245c/ which sadly I don’t see having earned an EnergyStar rating: https://www.energystar.gov/productfinder/product/certified-water-heaters/results unless you can find it. No clue how up-to-date the Fed Gov’t keeps this list. And sometimes there are good reasons for not getting EnergyStar rating, e.g. if the criteria aren’t keeping pace with technological changes and/or consumer preferences.