There are huge differences in tankless style water heaters and the fine print is very important. I have never seen a unit listed as "outdoor" that was safe to use in a closed space so please... do be careful.
Beyond efficiency ratings, be sure to take a look at (or ask) what it's altitude rating might be. Nearly all tankless are only good to about 5000 feet and fall off quite quickly well before that. OK unless you are camping in the mountains but above 5K it will not produce any hot water. Some have an altitude adjustment that helps but the makers often don't even mention it.
But proper venting of the CO2 is critical. Waking up dead really sucks.